Why Are Some Brands' M Sizes Smaller Than Other Brands' S Sizes?
Pet clothing sizes like S and M vary widely between brands, so you need to rely on your dog's measurements and each product's size chart instead of the label.
Different pet clothing brands use their own measurement rules, fit models, and fabrics, so labels like S and M are not standardized at all. That is why one brand's M can hug like a tiny tee while another brand's S drapes like a cozy blanket on the very same small dog.
Picture this: you order a raincoat and a sweater for your little Yorkie mix, both marked "M," and one slips on like a dream while the other stops halfway over her shoulders. It is such a common headache that multiple pet brands now invest in detailed size charts and even live "fit experts" just to help pet parents avoid returns and wardrobe fails. The good news is that once you understand how brands really build their sizes, you can ignore the letters on the tag and choose outfits that fit your pup snugly, safely, and comfortably every time.
The Big Secret: S, M, and L Are Just Brand Nicknames
Behind every cute "S," "M," or "L" tag is a set of actual numbers: chest girth, neck girth, and back length, sometimes with weight ranges mixed in. Fitwarm explains that a basic size chart is simply a map between those letters and specific measurements, and that each company builds its own map around its preferred "fit model" and target body type. That means there is no universal agreement on what "M" should measure.
Several size charts show this clearly. One company uses even-numbered apparel sizes like 8, 10, and 12, each tied to exact back lengths and chest ranges in inches. Another offers XS through XXL but bases those letters primarily on back length ranges, not on chest at all. A third uses numbered apparel sizes 1 to 7 with detailed chest, neck, and back measurements, plus separate XS to XL labels for harnesses and collars. When every company chooses its own system, there is no reason their M and S labels will line up.
This is why a small-breed dog with about a 10 inch back and 15 inch chest might land in a size 10 on one chart, fall into the "Small" band on another, and be matched to yet another label entirely at a company that prioritizes chest girth over length.
Why One Brand's M Fits Like Another Brand's S
Different Measurements Come First for Different Brands
Some brands start with back length, others start with chest, and some care most about neck girth or even paw width for shoes.
For example, some coat lines use size numbers that roughly match back length in inches and then allow a couple of inches of back-length wiggle room. Major measuring guides emphasize that you always need back length, chest girth, and neck girth together for sweaters and coats because chest fit affects breathing and movement. Other brands size certain coats strictly off chest girth ranges.
If Brand A builds its whole grid around back length and Brand B builds around chest girth, the same dog can fall into completely different labeled sizes. That is how Brand A's "M" can be cut for a longer but narrower dog, while Brand B's "S" is roomier through the chest for a stocky Frenchie or Pug.
Unique Fit Models and Body Types
Fitwarm notes that fashion brands, including pet brands, grade their sizes from specific fit models and target body types, then treat that internal sizing logic as part of their identity. For dogs, that could mean:
A slim, long-bodied "fit model" such as a Mini Dachshund or Italian Greyhound, which leads to leaner chest measurements for each label, as you see in some charts where a 12 inch back is paired with a relatively modest girth.
A barrel-chested or compact model, like a French Bulldog or Pug, which pushes chest numbers higher for the same back length. Some guides explicitly call out barrel-chested and long-backed dogs as needing extra chest room or extra back length compared to other breeds of similar weight.
Because each brand chooses its own base dog and then scales up and down from there, a label like "M" can be tailored to a narrow, long silhouette in one brand and a broader, shorter silhouette in another. On a small mixed breed with a short back and chunky chest, that can flip the experience so that an "M" from one company actually feels smaller than an "S" from a brand built for broader bodies.
Fabric, Stretch, and Intended Fit
Sizing is also tied to how the garment is meant to feel on your dog's body. One patternmaking guide shows that stretchy knitwear and non-stretch woven fabrics need different "extra room" built into the pattern. Knit sweaters can be cut closer to the body, relying on stretch to allow movement, while raincoats and down jackets need more built-in ease.
Some experts also warn that non-stretch pieces like jackets, vests, and dresses allow less flexibility and often require choosing the next size up. Other designs use snug, elastic coats that are intentionally body-hugging; the elastic panels stretch as your dog moves and trap more warmth.
So a snug, elastic coat in "M" might have smaller chest numbers than a loose, blanket-style raincoat labeled "S" at another brand that is designed to drape. You are not imagining it; they really are aiming for different silhouettes and comfort zones.
Psychology, Marketing, and Why Brands Avoid Comparison Charts
Fitwarm draws a parallel to human fashion and points out that "vanity sizing" and marketing psychology also shape how labels are used. Brands see sizing as part of their identity and intellectual property and rarely publish honest cross-brand equivalence, because that would expose who runs small, who runs big, and who quietly relies on flattering labels.
The article notes that, in human fashion, size and fit problems account for the majority of online returns, and that detailed, data-driven size charts can cut return rates meaningfully. But building and maintaining cross-brand comparison charts is expensive, complicated, and risky when bodies and fits are so diverse. The result for pet parents is simple: you are left to translate every brand's A, B, C into your dog's actual inches yourself.
How to Decode Sizes So Your Small Dog Actually Fits
Start with Real Measurements, Not the Tag
Multiple measuring guides all say the same thing: always measure your dog before you shop, because label sizes differ wildly by brand and even by product line.
For a small breed, three numbers matter most:
Chest girth, measured around the widest part of the chest just behind the front legs while your dog stands. Several guides call this the most critical measurement for clothing and harnesses because it affects breathing and shoulder movement.
Back length, from where the collar sits at the base of the neck to the base of the tail, with your dog standing straight. This is crucial for coats and sweaters so they cover the back without interfering with bathroom breaks.
Neck girth, around the neck where the collar naturally rests, using a "two finger" rule so the tape is snug but not tight.
Many guides describe essentially this same trio. They often suggest using a soft fabric tape, measuring after a walk when your pup is calmer, double-checking each number, and then saving them on your cell phone so you can shop confidently online or in a boutique.
Match Numbers to Each Brand's Chart for That Specific Item
Once you have the measurements, ignore the letter on the tag and look for the chart for that exact product. Nearly every brand warns that different product types within the same brand can size differently.
One outerwear line uses body measurements for coats but paw width and length for boots, and may even provide separate breed-based suggestions if you truly cannot measure. Another company has one chart for apparel, a different set of measurements for harnesses and step-in harnesses, and yet another for collars. Several retailers stress that you must compare your measurements to the individual chart on each product page rather than relying on a single "brand size" or on what worked last time.
A practical way to think about it is this: for each new garment, you are checking whether your dog's chest, neck, and back fall inside the ranges on that chart, regardless of whether the column is labeled XS, M, or 7.
When in Doubt, Size Up Strategically
Many fitting guides converge on one simple rule: if your dog falls between sizes, choose the larger one. They consistently recommend sizing up when the measurements sit on the upper edge of a range.
There are a few nuances you can use to fine-tune that decision:
If the item is non-stretch (like a raincoat, down jacket, or woven shirt), some pattern guides suggest leaving more extra room around the chest and neck, which favors sizing up.
If your dog is deep-chested, broad, or very fluffy, several brands note that chest girth can push you up a size even if the back length seems small on the chart.
If your pup is long-backed but slim (think Dachshund or Corgi), some manufacturers indicate that back length may be the deciding factor, especially for sweaters and coats that need to cover the spine.
If the clothing is specifically designed to be snug and stretchy, like many knit sweaters and close-fitting coats, the chart may intentionally look smaller; in that case you match measurements carefully but expect a closer, hug-like fit.
For tiny breeds, where a difference of just an inch in chest can completely change comfort, this "size-up when unsure" rule can mean the difference between a cozy romp and a wiggly, sulky dog trying to escape a too-tight outfit.
Get Help for Hard-to-Fit Little Bodies
Some small dogs are all chest, others are all legs, and some are petite but fluffy. That is where brand support becomes gold. Some retailers offer live chat and even back their sizing advice with a replacement guarantee if their recommended size does not fit. Others encourage customers to reach out with breed, weight, and measurements so a human can interpret the chart.
Because each brand knows its own patterns and intended fit, this is often the fastest way to figure out why, for your dog, their M is effectively smaller than another company's S, and which size will actually feel right.
Simple Example: One Dog, Three Different Labels
Imagine a 9 pound Maltese mix with a 12 inch back, 13 inch chest, and 9 inch neck. A chart that focuses on back length alone might steer you to a size labeled S based purely on the 12 inch back. A brand that ties both back and chest into its sizes might treat that combination as a size 10 or 12. Another brand using broader chest ranges for the same back length could group those measurements under an XS or even XXS.
All three garments could technically fit the same dog, but they would arrive with three different letters or numbers on the tag, and one company's "M" could absolutely be smaller in actual inches than another brand's "S."
FAQ
Can I Choose the Same Size Across All Brands Once I Find a Good Fit?
That is tempting, but the research from Fitwarm and other measuring guides all says no. Because every brand uses its own fit model, pattern grading, and intended ease, the safest approach is to treat each new brand as a fresh puzzle: reuse your dog's measurements, not the old size label.
Is Weight Enough to Pick a Size for My Small Dog?
Weight alone is not reliable. Fitwarm highlights how very different dogs of similar weight can have completely different chest and back shapes, and other guides echo that measurements should always come first. Breed- and weight-based suggestions from general size charts are best used as rough starting points, then confirmed or corrected with your tape measure.
Why It Matters
Size letters may be confusing, but your dog's actual inches tell the real story. When you measure carefully, read each brand's chart, and favor comfort over the letter on the tag, those "how is this M smaller than that S?" moments fade away, leaving your small pup free to strut, zoom, and snuggle in outfits that fit just right.
References
- https://shop.akc.org/pages/the-ultimate-dog-apparel-measuring-guide?srsltid=AfmBOopjL2oxYScVfKXAh2p1uHcw-1yK2vGRlnRvpA0AWTcVo8kMrh7w
- https://millamilla.net/pages/size?srsltid=AfmBOooDjX5u3EOhqB30oUcQiArN5gGM7V87RVRZTA3e2MBz5eJFhjg5
- https://fuzzyard.com/size-chart?srsltid=AfmBOoosPdmHG2r993L_LfTJ_-Qw1DsS-HkeoMa4KBflK2hIuteg0dL0
- https://alternativecaninetraining.com/what-size-dog-is-the-best-fit-for-your-family/
- https://beanadesign.com/pages/size-guide
- https://ctdoghouse.com/choosing-the-right-size-dog-for-you-a-guide-to-small-medium-and-large-breeds/
- https://www.funnyfur.com/how-to-measure-size-guide/?srsltid=AfmBOoozEUg1Q4tkyzsuLybqQIu502ZkpQ_nn5PaaNz6Fxj_vWXTbF7J
- https://gfpet.com/pages/gf-pet-sizing-guide-size-charts?srsltid=AfmBOorC50Zr6FAXLfi6As7msbwulBXmWVewxjHffpt9Ba6lnP8tX7Nz
- https://hudsonhoundstooth.com/pages/apparel-size-chart
- https://www.lambwolf.co/pages/soho-size-guid?srsltid=AfmBOorN7vP0vGFsPR17Uhc7GYIKjB67UbSREwCMav9_z8AXRlIKbrXz
