I just had what I think is an interesting observation while I watched my children playing dress up.
My daughter always dresses up as a "pretty princess" or a "beautiful ballerina," whereas my son dresses up as spiderman, batman, a cowboy, or a knight.
It is not enough that a little girl be something (i.e. a ballerina), even as a young child, she must live up to certain beauty expectations while doing so. I am not trying to argue that men do not have pressure to meet certain standards with their appearance as well, but at least a little boy can dress up as a cowboy without having to be a "handsome" or "muscular" cowboy.