My little guy noticed it when he was about 4. He asked dad if mom was fat, and my husband just about died. He kept telling our son, "don't ever call anyone that", over and over. I was surprised at my husband's reaction, and really ashamed that he had to defend me over fat.

My son will never call anyone fat though. Which is a good thing.
Yesterday, one of my neighbor's little guy called another little boy's mom fat. His mom was in shock. She went and had the boy pick some flowers and give them to the overweight mom and apologize.
It's such an innocent observation when they're young.