Its so hard to say what a heavyweight would be like if he was small.
He would be a completely different fighter.
I always look at P4P as a COMPARATIVE ranking, that is to say how he ranks compared to other fighters in his weight category.

Roy Jones Jr. is more dominant, or shall we say superior to a larger degree over fighters in his weight category, than a given heavyweight is better that the fighters in his division.
You can't compare a middle to a heavy, head to head, it just doesn't work so you compare them to their peers.
Thats how I see it anyway..
Of course the best way to quantitatively judge a fighter is on how he has done in his division, IE his record. The fighter in ANY weight class with a record that is head and shoulders above any other fighter regardless of weight division must clearly be rated as the best P4P fighter especially if he is undefeated.
Say if a guy had a undefeated record, a long winning streak, a lot of title defenses, maybe setting or breaking some kind of longstanding record of like Rocky Marciano's 49-0 It would be VERY hard to argue that he wasn't the best P4P in the sport. Wouldn't it?