Everybody has noticed by now the phenomenon of vanity sizing, wherein the size numbers printed on the tag shrink over time even though the clothes have stayed the same size. It's a phenomenon of our own doing. People want to remember themselves as the size they were when they were younger, and fashion designers are eager to please. I remember reading an article about this phenomenon a few years ago that quoted a woman searching for a dress as saying, "I'm going to keep trying on size fours until I find one that fits." She was a size four, gosh darn it.
I know people who have to shop in the "Young Misses" section because the clothing in the "Women" section is too big. They haven't gotten any smaller, but the clothes have gotten larger. I used to wear a medium; now I have to get a small, and my bathroom scale will assure you that I'm definitely not getting smaller.
The article suggests that sizes will stabilize thanks to online shopping. In the store, you can try on an article of clothing to see how it really fits; when buying online, you don't have the luxury. The size number had better be accurate or you're going to be returning it. At least that's the theory. I'm just waiting for the madness to end.