I really don't think there is any advantage per-say, it's just how Paul Williams fights. I feel the guy is a freak due to having his frame at that weight, not remotely because of his ability to use it in the ring. He gives up his height completely, and rarely gets close to full extension on his punches, he has basically no functional reach advantage over good fighters. If he had a different style I think it would be ridiculous, but that's purely hypothetical going by his physique.