Why do girls always say to other girls "You're so pretty, beautiful, gorgeous" when they clearly aren't?

I've noticed a lot of women have two modes - "destroy her" and "build up her esteem". We are squirrely little creatures that way. I am an esteem builder, but I am also painfully honest. If a friend asks me how she looks, I'll tell her she is pretty, etc, because I feel all women are beautiful in their own way. However, if she asks me if her trousers are making her look fat I'll say something more like "I'm pretty sure it's your fat making your look fat". I do not have a lot of female friends.