OK. I'll jump in. I'm 5'6" and currently 140 and I generally wear a size 10, although sometimes a 12 depending on the brand, and generally a medium. Athletic clothing I need to buy larger--always a 12, or a large or XL.

Here's the thing: when I was in junior high and weighed 110 pounds I wore a size 10. In college, when I started rowing and building muscles I was up to 120 pounds and wore a size 10. After college I started cycling; when I got married I weighed 130 and wore a size 10. I put on a lot of weight after my second baby and only when I was 150+ did my size really go up--definitely needed a 12. When I started needing a 14 I decided enough was enough and I lost weight. I was back down to 130 a few years ago and STILL wore a size 10. When I started training for triathlons, I got back up to 140 (funny, I just realized that every new sport I take up causes me to gain 10 pounds). I tend to think of myself as the poster child for vanity sizing--I get bigger, but my size doesn't change! I think athletic clothing doesn't give in to vanity sizing nearly so much, which is why I'm in a more honest 12, L, or XL.

Part of the problem with sizing, though, is that sizes like "small, medium, and large" are comparative. Compared to most American women I'm pretty much medium sized. I'm not tall or short, just average. I'm not fat or thin. Just average. So in a way it makes sense for those sizes to change over time as the population changes--I might be enormous by 1930's standards, but for now I'm just medium sized.

Sarah