Hmmm, I think that even if you are slender, once you go above the weight you feel comfortable at, you feel "fat" and it just bothers you. It's not that you are fat, it's that you are more than you want to be and it's just easy to use the word "fat" to describe it. There may be people just fishing for compliments, but some aren't and won't change the discomfort they feel just by hearing, "You look great; you don't need to lose weight!" (Though the compliment will likely be welcome.)
I know someone who is quite thin, and she doesn't like to hear about other people's diets because she is trying so hard (and unsuccessfully) to gain weight.
I remember my very thin mom not indulging in something extra (after a good meal, she never did any actual diets) because she didn't want to gain weight, and getting crap from people telling her stuff like "What do you have to worry about? You're already thin."She found that a bit offensive.
I dunno. Lots of people are sensitive about their weight, heavy, thin, or medium. Maybe the best thing is to not take it personally and remember that they are talking about themselves, not you. But yeah, I agree that it might be good to just say you don't want to talk about their weight anymore and that if it's a problem, they should bring it up with their doctor. That's a good one, actually.
"Somewhere along the way, someone is going to tell you, 'There is no "I" in team.' What you should tell them is, 'Maybe not. But there is an "I" in independence, individuality and integrity." Â George Carlin