Calling yourself fat is taken as an insult or a cry for help/ sign of body dysmorphia by those fatter than you.
Even when said in the most obviously lighthearted way, it usually results in a fatter bitch or guy to stop, look you in the eye, and say something like
"No. You're not fat, dude." In a serious, false reassurance tone.
Personally, I'm not exactly an example of health.
Papa has some chub on him
But I'm athletic *enough* to enjoy physical outdoor activities and do necessary physical labor without being a fat short of breath mess a few minutes in.
Still, I think being realistic with oneself is important when it comes to personal health.
So it's very funny to hear a "high-calorie person" try to convince someone they have no excessive fat whatsoever, for the sake of not accepting or acknowledging their own obese reality