After someone starts topping 200 pounds or so, the general public has a hard time determining just how much they actually weigh.
If I were to ask people to guess my weight, I doubt many people would get within 50 pounds. Granted, they might be guessing low just to be polite. But I think once you start to get really, really, fat, people lack a frame of reference to determine your weight. It goes so far off their weight radar that they will guess anywhere from 50 to 100 pounds off. Would they similarly guess a 100 pound woman weighed 200? Not unless they needed new glasses.
Vice versa I cannot tell when someone has gained only small amounts of weight. If a friend put on ten pounds, I would probably not notice. I just don’t perceive those things. Perhaps the lack of this ability contributed to my weight gain in that I didn’t notice when I myself gained ten pounds.
I saw an article online recently about celebrities who had recently gained or lost weight. Looking at the pictures, I could honestly not tell any difference. If I had to choose which was the “before” and which was the “after” photo, I would have gotten only 50/50 at best.
I think some people are just more focused on weight. I had a friend in high school who would always point out when someone had gained or lost pounds. She was a human fat detector. She was never satisfied with her own body either, making me wonder if her own self-obsession with weight was being projected upon others.