I feel that it is boxer dependent more than anything. Sometimes punishment is worse, for say meldrick taylor (after JCC fight). Other times it is an age thing, where Mike McCallum started to lose and be less effective as he got older. Some fighters like Glen Johnson actually improve as they get older, but I think most of those fighters had limited amateur careers and take some time to figure things out. Limiting punishment and staying in top shape can extend a career, as Hop n Floyd have proven. Both of those guys have great genetics also though, so I feel they are the exception and not the rule. The last two factors to me are weight fluctuations (up and down), and just rising too high in weight. Roy and Chris Byrd are examples of guys who went up and down at a point in their careers and seemed to lose something. Mayorga and Vince Phillips are examples of guys who were good at a specific weight (147,140) but ineffective at anything higher.