I guess this is a case of "don't speak about others injured limbs".
Kinda the way it is rude to stare or speak about huge scars on a person's body.
The asking or saying something may be harmless enough in intent, but there is also a good chance it is a sore spot of the person, and they do not want to speak about it. Especially with someone they are not intimate with.
Still, why should being fat be shameful? It isn't to everybody.
A good friend of mine is fat and uses the term herself. I can call her fat so long as no lecture or unkindness is attached.
But I never call her fat!
She already knows anyways.