Exactly.  Fats comment on you and make you feel uncomfortable (I sure as hell didn't like having my mother speculate that I'm anorexic), it's okay, but when you notice their obesity you're a horrible shallow fat-shaker who is being mean to people. It might be that on a subconscious level they know being fat is unhealthy and being physically fit and thinner than the American norm ("anorexic") isn't.  Psychology textbook Sticks and Stones points out that people feel shamed for someone innocuously noticing X if they secretly feel ashamed about X.  If they aren't ashamed at all of X, they cannot feel shame for someone innocuously noticing X.    Exhibit A: Fat Acceptance people are offended at being referred to as fat--and also offended by people avoiding the word "fat."