A fighter's ability regardless of weight class. You're proving my point. So, by that definition if a fighter went up in weight and won it proves that his/her ability was able to win regardless of the weight class. It's actual proof that their skills were enough to win even with a size disadvantage. If a fighter went up in weight and won, then it means that even if size was equalized they would win, most likely even easier since they no longer have any disadvantage. Manny Pacqiuao is heralded as a P4P great. Why? Because he went up 8 weight classes, proving that his skills could actually overcome weight classes. Roy Jones Jr could go all the way up to HW as a natural middleweight and still became a champion, another P4P great. Hell, sometimes people don't even have to win. Valentina is considered P4P GOAT material due to how incredible she performed against Nunes, someone that's far bigger. Another case of a fighter proving that their skills alone could transcend a weight class.
P4P is a skills based evaluation. What better to judge skill than having it overcome the greatest advantage in fighting; size.