Actually at the HW division the weight really doesn't matter anymore. Tyson was routinely outweighed by 15-30 pounds and fought guys that were 5 to 6 inches taller than him and he won easily in the 80s, but people still consider what someone like Duran or Leonard did going up in weight to fight a guy such as Hagler as more impressive because like Bilbo said, size matters a lot at the lower weights more so than at HW.

It's hard for me to explain the physiology of it but Bilbo did a great job in explaining about it in his recent posts.