In a 2003 study, Tammy Kinley, a professor of merchandising and digital retailing at the University of North Texas, found that size 4 pants varied by up to 8.5 inches in the waist, and size 6 pants varied by up to 13.2 inches in the length of the crotch seam. So, no, you're not imagining those disparities.
Brands develop sizes to fit the women who buy their clothes—or sometimes to fit the women a brand wants to buy its clothes. "High-end brands tend to size smaller; their size 8 is going to be smaller than a mass-market brand size 8," says Kathleen Fasanella, a patternmaker and the author of The Entrepreneur's Guide to Sewn Product Manufacturing. Furthermore, some brands that market themselves as contemporary are designed to fit young or college-age women, who are typically straighter in the hips than they will be later in life. And most major brands have now gone international, so you'll see European sizes on labels today.
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And then there are those jeans and pants sized by the waist measurement: 28, 29, 30, and so on. Sounds like a move in the right direction, right? Well… For a new study that she is conducting, Kinley says, "Just today I measured size 27 pants in the lab that were 32 inches in the waist. Even when the system gets more specific, it's still arbitrary."
Here, a look at how vastly a size 10 varies at 11 top brands, so you can finally find clothing that actually fits like it should: