We’re conditioned to believe that fat is bad
When I was at my fattest, no one mentioned it. If I mentioned it everyone said 'you're fine' and 'it's okay because you're healthy'. Everyone bent over backwards to fuckin' lie to me, or they were lying to themselves on top of it because generally I was also smaller than them despite being disgustingly fat and completely sedentary due to depression.
My skinny as fuck brother even lied to me. After criticizing someone else's laziness to me, I pointed out I am exactly like that person. I wasn't proud of it or trying to corner him; a part of me very deeply wanted him to call me out on it and tell me to stop being so fuckin' lazy and irresponsible. He immediately backpedaled. "Oh but at least you (insert thing here that has no bearing on the discussed topic)."
We're conditioned to not want to hurt each other's feelings as much as possible. At least most of us. Assholes will be assholes. So if someone who is your friend, loved one or other person you usually value and they actually say something to hurt your feelings on purpose, you know they mean it. It's not social conditioning; they're going against their social conditioning.