Why do skinny people often claim they eat a lot when it's obvious they don't eat that much, and vice versa for fat people?

A lot of skinny people claim they that they eat like a horse or whatever but are just naturally skinny. But like 9/10 if you are around them and you actually see how much they eat, it's much less than they say they do. And for fat people, it's much more than they say they do. So why do people claim this? Are they lying, or do they genuinely not realize how much they're eating?