I have a few friends who are/were pro MMA fighters. All are very big dudes, but without a doubt the bigger ones gas out faster just because it takes a lot of gas for big guys to move, and they have to work harder at it IMHO, whereas the smaller dudes have all this energy, not as much power on their feet, but on the ground, oh man. They can wrestle for hours, I've got maybe 10m before I need a break. And I'm reasonably fit (I do a lot of gardening and outdoor heavy lifting as well as strength, cardio) but man it sucks even having an extra 20 lbs.
Lucky for me I'm a middle aged man so all that extra weight kind of finds it way to my abs. A little tanky bod is OK by me. But yeah, daily fitness routine is a must for anybody, but especially martial artists. I'll never understand all these YouTube masters who want to teach me kung fu secrets, but look like they eat BBQ for every meal and never take a walk.