At the very highest three or so weight classes this is true, but for the vast majority of classes you will find very fit people who diet and train very hard to get into or maintain their desired weight class.  Source: I have bench pressed over 500 lbs in the 220's, 242's and 275 weight classes, and am training right now to compete at 198.  I've not seen any fatties who were competitive at a high level in any weight class below 242, and even there it's rare. Even at 275 and 308 the top guys are surprisingly lean, and there are a few super heavy with visible abdominal muscles on those huge guts (see Scott Mendelson for example).    But there are way too many fatties out there who are claiming to be powerlifters while not putting up lifts commensurate with their weight class, no doubt about it.