I think that the only thing they really change on women's shirts by size is length and (once you get up to the X's) room for bust.
I can wear size S a lot because I'm short and I don't have a lot in the breast department. (though I prefer men's size M so I don't have to worry about the length or my stupid shirt riding up; unlike a lot of girls in my area, I don't like to show much skin)
I think the general aim of all clothing companies is to make us feel fat when we're obviously not.
But maybe I'm just paranoid.