Why do some women say things like "geez, you are soo skinny!!! Tiny"?

I've had some girls accuse me of being anorexic, and they always say things like "you're too skinny" so casually like it's a compliment. If I were to reverse this statement and say something like "god you're so fat!! Huge!!!", it's considered an insult, but for some reason constantly being told "too skinny" is not.. What gives?