Well with your example of Holyfield that is easy to see. Holyfield was in fact the very first to incorporate a "modern training regime". His methods were considered radical, yet now they are common place. What's more when you look at Spinks, he was incredibly weak chin wise (Tyson KO1), as soon as he faced a puncher who could land on him properly unlike Conney he was finished. Except Holyfield has a thick solid heads and slabs of muscle. That's the difference, his chin is sound.

David Haye is an example of how modern training has enabled him to carry his power from CW up to HW!

As far as Holyfields power goes he was somewhat punch weak at HW and really wasn't a complete power puncher even at CW. His KOratio against HW's is something around 30 odd% which is telling and most of his KO's came in later rounds too.

Yeah shorter arms I believe can deliver more power "in closer" than a taller guy trying to throw shorter shots, it's the torque they can better generate, particularly in hooks and uppercuts. But the taller guys produce better power out long, especially with their straight shots. Since the outside game has become more important in modern boxing there is the reason why we are seeing more basketball sized boxers now.

Taking your Joe Louis analogy, that is a bit silly. Joe Louis was bigger than anything better and better than anything bigger. You see for a start, like Ali, Louis outweighed most of his opponents, over 80%! The ones who were larger like Buddy Baer, Ade Simon and Primo Carnera were absolute oafs! The Nicolay Valuevs of the 30's HW division except far smaller. Back in Louis's day there were no superheavyweights with the correct balance or skills. Today the big guys and the technicians are the same people! It is not easy to develop the balance and dexterity for boxing when you are 6'5"+ and 240lbs+, it requires maniacal training.

You keep calling modern opponents fat. Ali was fat, Holmes was fat, Foreman got fat, Frazier was chubby even in his "prime". Who is fat today? or recently at top level? Chris Arreola? He's getting fit again now. Sam Peter? Sure but when he fought Klit he was chiselled like a bodybuilder. Hell they even called Lennox Lewis fat when he came in 5lbs heavier than usual for his fight against Vitali! There is a massive exaggeration of the fat levels today which are frankly no worse than yesterday.

All pro boxers must train hard and long, otherwise they would get their head punched in.