You see underdeveloped bodybuilders lift big weights because they don't actually know how to lift with their muscles - that's why they're underdeveloped. It's why 175lb. powerlifters bench 405lbs. Their CNS / skeletal system / tendons / etc. are well-trained but their muscles aren't. This is the majority of guys in any gym - they train movements, not muscles.
Nobody comes out of the womb at 250-300lbs. The biggest guys got that way because they kept lifting more and more weight.