Why don't some people realize they're fat?

I've noticed that some overweight people seem to think their girth is "healthy," "muscular," or "average." Even obvious signs of flab hanging off of their bodies doesn't keep them from saying embarrassing things about their own weight. There are even a lot of questions on here asking "Am I fat?" and when they describe themselves, it seems blatantly obvious that they ARE fat. How could they not know?