:D You responded!
My thoughts are — if they don’t have body dysmorphia, they’re sometimes fishing for a compliment, along the lines of, “No you’re not! ( as an affirmation of their thinness and self-esteem, not as an affirmation of not being lazy, gross, etc.).”
Mostly I want to slap them because what I immediately think is, “If you think you’re fat, what do you see me as? Incredibly fat? You have no idea what it’s like to actually be fat, or you wouldn’t say that.”