Were Rahman, Peter, and Stiverne superheavyweights?

Any boxer who gains weight is gonna be stacking muscle and fat on their upper body. The muscle helps. I doubt the fat does much.

 

Guys like Ruiz, Toney, Foreman, Peter, Tua and Miller all had thick necks which likely helped then endure shots too as opposed to people like Wladimir Klitschko and Deontay Wilder. Mercer and Tyson had thick necks too, George Foreman has mentioned this theory a few times, of course that doesn't mean they still can't be dropped.

As for Jarrell Miller, he was juiced out the eyeballs and of course added fat but realistically he actually pulled off 315 pounds very good. The only fighters I've ever seen be anything absurd while having lean muscle was freak show MMA fighters Bob Sapp who was 6'5 370+ pounds with abs that got kicked out of the NFL and 7'2 Hong Man Choi who was 360+ and was in much better shape than boxer Nikolai Valuev.

 

I don't know if there is a source, but to my mind, even just being fat does actually help take a shot better, especially to the body. Sometimes with a fat guy though, it's hard to determine how much muscle mass he has, because it's hidden.
Butterbean for example was extremely fat, but he was probably carrying a fair amount of muscle mass too. Anyway, Bean could take a shot.
My way of thinking is that a punch is kinetic energy, and once that energy hits a target, that energy sends a sort of shockwave through the target. The more mass there is to absorb that energy, the better. I've never subscribed to the idea that fat is useless in a fight.
Of course, being insanely obese is one thing, but a very fat guy can still fight, and fight well. We of course have many examples of this.

Edit: Sorry, I see this has already been talked about.

As for what constitutes a 'super heavy', I've always thought about guys in the 6'5+ range, with appreciable mass, around 240+.
A Tony Tucker had the height, but not the mass. He was tall and lanky. Rahman has the mass, but not the height. He's just thickset.
I think a Super heavyweight needs to have both. IE. A naturally very large man who doesn't have to do anything like weight training for bulk to achieve his mass.