That's weird that all of you are saying you experience the opposite from expensive VS cheap. I wear a comfy 8 at the Gap, Express, and New York and Company. Certainly not expensive designer jeans, but far more expensive than my cheap Walmart jeans, which are a snug size 9 (I know it's a Junior size, but I figure a 9 is like a 10 with slightly less hip room). I'm a size 10 in some of the cheaper jeans carried by JC Penny, too. Possibly still a 10 at Old Navy? I haven't been there since I fit into 12s, and they're big on me now.
I always thought it was weird that sizes vary so widely, and came across some articles and people's personal experience stories stating that higher-end brands tend to run bigger. However, most of them mentioned shops like Ann Taylor Loft (where one woman commented that she was a 6 everywhere else but was a 00 there!), which I think are geared toward older women...so maybe it's only that kind of 'high-end' clothing? The stuff geared toward the upper middle class middle-aged woman.
I guess it would make sense that super designer jeans would run small...they have a certain demographic they want to fill, after all. So I probably wouldn't fit into True Religion or Seven jeans at all, thinking on it. xD As I said before, companies create their sizes based on the demographic they want to market towards. That, at least, is a proven fact.
Hence why it was such a big deal when H&M added an extra size. I think they squeezed all the way up to an XL and began a really cheesy campaign for a 'plus size' model. You could see their applicants, and half of them looked pretty petite.



