Size has to be proportionate to the rest of the body - it has the begin and end at the right places. It can protrude, but not too much or not too little. Guys are very interested in pronounced posteriors but there's a goldilocks zone. If all the metrics are out of the goldilocks zone, we know. And the implant stuff is no good - better to be out of the goldilocks zone than to have anything fake. Then there's the issue of cellulite, but that's a conversation for another time.