Why do people comment?

My son is genuinely very skinny. He was 8lbs at birth and is now 11.3lbs at 15 weeks. He is longish 24 3/4 inch. People comment on how tiny he is alllllll the time. And it really hurts my feelings. Like the way they say it is asking if there's something wrong with him. There's not. His doc says he's the healthiest baby he's seen in a very long time. I don't get it, I also feel like most babies are totally obese nowadays but I don't comment on it!!! I don't go up to someone and say "Jesus Christ that baby is huge you should feed it less" so why is the opposite ok for others to do?