slim = beautiful seems to me to be a standard mostly enforced by women on other women.
I was brought up in a household in which it was considered fine to comment on weight daily, if not more, and I have to say the impact on my self-esteem was hideous. Constant unfunny jokes and comments on my vast size 12 figure and round face. Really not helpful as I had a perfectly healthy teenage appetite (oh except for the eating disorder I developed at 14 )
then I gain all the weight back, leave home and go to university and people's (partic men's) perception of me and my figure seems very, very different.
I've never known a man to comment on his partner's or anyone's 3 kg weight gain (except for my gay friends, who seem similarly anorexically obsessed about themselves and others as a lot of women are). It's really sad and I think teenage girls in particular really suffer from this competitive control obsession.