Some brands even carry 000 now - no lie! I have often wondered the same thing as Mo. I am 5'10" and almost always wear a size Small these days. I used to be a Medium at my same weight. Sometimes I even have to get an XS. I wonder, "What about the women who are 5'2" and really tiny?"
I think a lot of women are excited to wear the smaller sizes. I have to admit that I do get a bit of a rush when I have to grab a size 4 instead of my usual 6-8, but I know it's not "real." I mean, if I lost weight and really needed a smaller size, that would be different, but I am not thinner or smaller; the clothes are just bigger!
Sizes are strange. I have been trying to find workout pants for my tall and thin husband for MONTHS! Manufacturers all seem to think that tall men are also BIG (you always see "Big and Tall," not just Tall). I ordered him a Large Tall from a big and tall men's store and the pants were HUGE! When I returned them, they said that their Large (their smallest) size is a size larger and more like an XL. Go figure! The search continues... I have the same problem being tall and slim sometimes. Some manufacturers start their Talls at Medium, not Small. I was always a Medium before, but not anymore. The tailor is my friend, and becoming more and more so...