Skinny/Fat stereotypes?

I just don't understand. I have been skinny all my life, always will be (hopefully at least). But it really does get old when people are always like, "Dang, your really skinny" "Are you sure you eat" and things like that. It gets old quickly. I know MOST people strive to be skinny (not hating on overweight people or saying everyone wants to be skinny just saying), but how would people like if I just went up to them and said "Dang, your really fat". Doesn't have the same connotation huh? I don't know, I mean I like being skinny and all and would like to put on some weight, but really, don't talk about how skinny I am and I won't point out your fat. Thanks