Who has mentioned about anyone punching harder, Jim? We are talking about fighters being bigger and stronger for which they are, better diets, better knowledge regards training, more advanced supplements (steroids?) and the rest of it means heavyweights are a lot muscular, physically stronger, heavier and leaner than they ever were.
Punching power has nothing to do with strength more than it has to do with timing and speed, Jimmy Wilde had very little muscle but was a supreme KO artist.
At 6ft 6ins and 18 stone - Primo Canera was seen as a circus freak in his day, now he would be bigger than average.

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If strength doesn't - as you agree - then why mention it? Marciano was able to smash much bigger men than himself to bits, (he once annihilated a 255 lb man, as did Dempsey, whereas Valuev (relatively speaking) couldn't knock the skin of a rice pudding.
I mean if you think about it, what advantage does all that extra size confer, apart from reach - ? Wlad for all his size, can't hold a shot anywhere near as well as Ali or Frazier, or Marciano for that matter, all smaller men.
I think the fixation with size at heavyweight is slightly misleading, what people forget is that once guys get beyond a certain size, they start to become less agile - this isn't a factor below heavyweight, because these are 'normal' sized humans - anyone 6'6" and over is basically pushing the limits of how big a human can be, and still be capable of athleticism - Wlad is fairly nimble for his size, but he's not exactly balletic in there, nor could he ever be. If size was everything, then Valuev would have been unbeatable, but clearly, he wasn't, and he struggled with a very old version of Holyfield, who many thought was robbed of the decision.
Let's not forget, really big men like Wlad, are still a rarity in the division, go look at the top 50 boxrec heavyweights, how many guys are there over 6' 4" - not many - most are the same size as Ali and Foreman - I still believe that 6'3 and 220-240 lbs is as big as any heavyweight needs to be if he has the skills, the problem isn't that Wlad is so much bigger than everyone else - as we shall see if Fury ever fights him - it's that he has better skills, and a better fighting brain, not to mention a wealth of big fight experience.