Earnie Shavers and Mike Tyson probably take this. Obviously Marciano, Frazier, Patterson, Langford, Louis. Weird honorable mention to Deontay Wilder, who hit harder than probably all of them while being small in terms of weight (obviously not height). Another mention has to go to James Toney. How many natural middleweights can get fat, campaign at HW simply because they got fat, then stop a consensus ATG chin? I’d bet there are some LHWs and Cruisers who could enter the conversation if they moved up. Can’t imagine that Beterbiev or Opetaia wouldn’t be harder than average hitting HWs. this is a large category though. Really we’re just excluding super heavies.
I already read all the majors, Marciano, Dempsey, Fitzs etc. ... hard to not be redundant so if I am, my bad, tried to avoid it. Barbados Walcott is the smallest man I know of who has floored an over 200. Barbados was 140 Haye's given up the most weight in a championship match and won, I think. Like 100 pounds wasn't it? Tommy Burns is criminally underrated, especially as a good small HW.
Today that would probably be a Light Heavyweight entering the ring at 190 pounds. And I agree that it might be possible for some of them to stop some Super Heavyweights today, if given the opportunity. We just do not know for sure.
Wilder could have fought at cruiserweight if he wanted. He could have drained to 199 pounds and came in at 212-215 on fight night.
Beterbiev could probably do it (or could have when a bit younger). As you point out, probably actually only has to bulk up by 10lbs to be able to make HW. Would he be stopping Dubois? Highly doubtful. Could he maybe stop some of the second tier super heavies? Possibly. Journeymen super heavies? Would be hilarious to watch him blast them out. Maybe fight Jake Paul at HW? Why not? Would be a one sided destruction. Hell, he could walk in at 175, let Paul weigh whatever he wants, and still do it.