Because they don't want to think they're as fat as they actually are. It's just muscle and water weight teehee. They don't want to face reality and want to think that half-repping some "heavy" weight "proves" they're not a fat fuck. A shitlady friend of mine got shoulder-checked by a fatty (seriously she's so hot that she pisses off fat people by existing), but since she lifts well over bodyweight at around 110 pounds, the fatty didn't even faze her. You could see its spirit broken in its eyes.