How big would heavyweights be today if fights were still 15 rounds?

And conversely, how big would heavyweights have been in the past if fights were always 12 rounds?

Considering all the discourse that goes around how fighters in the past would stack up against the modern day super heavyweights, it seems like the 3 round difference in fight length between the modern day and the past isn't addressed as a cause nearly as much as it should. After all, there are guys like Joshua who has said he's not designed to go the distance even in the 12 round era who've found great success regardless.

The rapid rise of the super heavyweight came in the 90s, pretty shortly after the end of the 15 round era, and even with a guy like George Foreman, you can see how much more he weighed in the 90s than the 70s, although you can probably chalk that up to age more than anything else. If Muhammad Ali fought in 2010, do you think he puts on another 10 or 20 pounds knowing that he doesn't have to dance around for 15 rounds? Or do you think he would be entirely replaced by another guy with a naturally bigger build (I doubt many of the people on this section of the forum will go with that option. Maybe I'll pose this question on the modern side of the forum as well).

And do you think a super heavyweight would be able to go 15 rounds with an Ali or Louis and not gas by the end? Or would the modern day cruiserweight just become the heavyweight champion of the past (obviously, Usyk would probably be the best heavyweight in the past cuz he's the best heavyweight now while being a naturally cruiser anyway, but put that aside for now).

Hell, if fights were always 12 rounds, maybe Primo Carnera would be remembered as the GOAT. Perhaps all of the fighters we love would've been outclassed by bigger dudes. I don't know how much the round change makes a difference compared to diet, drugs, and genetics, or even the movement of athletes to other sports, but I am curious about why the best heavyweights always fluctuated at around 210 when there have always been much bigger dudes out there even in the distant past.

As an aside, do you think this affected the guys at lower weight classes as well? Obviously the guys are fighting at fixed weight classes, but are naturally smaller guys fighting at bigger weights because they care less about their conditioning and more about putting on muscle?