The guy has a small frame, but it does look like he has a lot of muscle for his size. The powerlifts are a lot about leverage too, and he has good leverage for deadlift especially. If he gained a lot of weight, I am not sure how much the lifts would go up. It probably wouldn't be proportional though. He has found a place were he is strong while being as light as possible, which is important for a weight class sport.