JDS tiring is not a weakness of his. When you are that big it's insanely hard to have good cardio, so much to the point that Cain is a freak of nature, literally. Gain used his strength (nearly endless cardio) to beat Junior, in fact Junior being able to even last 5 rounds and continue to fight off takedowns showed incredible stamina. Cain's strength is not Junior's weakness, he is the combination of good genetics and an insane work ethic.
The fight was not a revealing of Junior's weakness but a representation to everyone how far Cain actually push himself, Junior impressed a lot of people in the fourth and fifth by even attempting to get a second wind and fight off takedowns. Nobody else has been able to keep Cain off them like he did even in defeat. I fully expected Junior to win because I figured there'd be no way Cain would get him down, he proved the world wrong, glad I didn't bet any money on it.