I'm not so sure about this. The Rock has only slightly more surface area than I do (due to his larger physical size); it's not a huge amount more, certainly not double. However, he has much more muscle mass than I do, probably more than double. Therefore, his surface-area/muscle-mass ratio should be lower, meaning he should have less ability to exhaust heat than me. (Muscle mass is what generates all the heat.) Therefore, over time, he should get overheated more quickly than me, though of course this depends entirely on our relative physical activity while we're trying to fight or avoid the other.