Heavyweights are a different animal than other weight classes. There weight and how they were able to use it was a big part of the weight class. Now move them into a weight class where there weight was not a skill anymore, but things like quickness, .etc are and that wasn't as well developed a skill as using your weight you can see they will be disadvantaged.
I have only antecodel evidence from HS where our heavyweight who was very good, lost a ton of weight and he was no longer as good. When he had his weight loss I was able to wrestle him. I noticed he was slow to move, unable to fight out of defensive positions probably because he never experienced them before.
Though he gained a lot in his personal life.