Yes age is relative and Pacquiao relatively fights younger opponents than Mayweather. Thats a fact.
Floyd beat a bigger Oscar but he also only beat him by a SD. A year late Pacquiao schooled Oscar with a masterclass then stopped him by not only retiring him on his stool but from boxing all together. Pacquiao also stepped up 3 weight in a year for this fight.
Hatton was in the fight with Mayweather up until 6th round. Hatton got frustrated and tired and Mayweather picked him off. Hatton at his preferred weight was destroyed in 2 round by Pacquiao, with devastating speed and power and accuracy.
Marquez pushed Pacquiao hard but he was comfortable at that weight. Mayweather made Marquez jump 3 weights in a year at the age of 36 and Floyd still came over the agreed limit and looked the biggest he has ever been. Marquez has always been poor again counter punchers and defensive fighters.

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