Why Is it frowned upon when someone calls someone else fat? But not when someone says "you're so skinny"! "Or eat more"!

Sabrina Rane
I take offense to this because I've been skinny my whole life and very insecure at the fact that I have small breasts and no bum. It annoys me that's it's alright in society to practically call someone anerexic but God forbid the words "you look fat" escape someone's mouth ? Please elaborate.