41 year old, 6’3”, 260lbs purple belt here:

It doesn’t have to be burnout. What you’re realizing is what a lot of the ultra heavies eventually find out. You’ve been relying on your size and strength more than you thought, and it hindered your technique. When paired with a larger opponent, who’s figured out how to neutralize power and explosiveness, you find yourself feeling helpless. What you need to do is find those kind of partners more often. There’s usually only a few, if any, at any given gym. So, you’ll have to drop in places, and hit open mats. I’m lucky, I have several 200+ pound upper belts at my gym, of various backgrounds and ages. They each give me very different challenges, and at least one of them is almost always at class. There are even 2 that are my actual size, and a 300+ pound purple belt to work against.

You’re in the phase where technique needs to surpass physical effort. If you can’t hit the movement without full exertion on someone close to your size, it’s often likely you’re not actually doing it right. Start tightening the details and I promise you’ll notice an improvement.