You can't be serious. You realize heavyweights don't diet, right?
Dieting isn't some mystical voodoo, anyone can acquire the knowledge with a simple Google search and the basic principle has never changed. Caloric deficit to lose weight, caloric surplus to gain weight.
As you said, Cormier has never lost a round at heavyweight. He's the champ. He has zero incentive to diet down and fight at middleweight. I'm not saying he should. I'm saying he could.
I love how you assume just because he's an athlete he's on a perfectly optimized diet. All you have to do is go find interviews of him talking about food to realize he eats junk food just like the average American. And that's why he looks the way he looks.