When you're talking about someone that is larger due to their physical exercise and weight training, the cardio threshold will be equally as high. This is why they establish weight classes. Someone who is a flyweight cannot stand a sporting chance against someone who is a heavyweight. The flyweight may have speed and good hits, but they stand to sustain significantly harder and more damaging blows from a heavyweight, with a great chance of injury and knockout. The physical advantage goes to the greater size and weight. Such is why you won't ever see Brock Lesnar fighting Ian McCall.
I may not be able to run 15 miles with an MMA fighter, but I won't have to if I have a significant strength advantage coupled with a capable aerobic capacity.