American football and basketball players salaries sky rocketed in the 80's which is a big part of the reason why Americas best potential heavyweights resort to playing football or basketball and the American heavyweights began to diminish.
Not sure if its just the impression that I get, but I think as a whole, the average human height seems to have increased. Aren't most kids taller than their parents? Probably better diet, better environment. People are just getting bigger. So boxing, and not just boxing, but other athletes, are definitely getting bigger along with the rest of the human race.
Diet awareness has come on in leaps and bounds since the 60's / 70's.
Athletes are now well informed with regards to vitamins and nutrients... This has enabled them to grow properly so to speak.
Today's heavyweights are bigger but yesterday's were much more talented.
Hevyweights today are just bigger and taller but definitely not better than their counterparts twenty years ago.
Heavyweights today are all (for the most part) "old", really tall and, usually never starts their careers before 21-22
Yesterday heavyweights were a bit shorter, and fought in there early twenty's and so on, it also wasn't uncommon for them to start their careers at 18.
