i dont know, but, looking through his record, there were some big dudes fighting back then. Bigger than i think people realize. Sure there were 190lbrs fighting heavyweight, but Jack was 225 vs the 240lb Jess Willard, and Jeffries was 220ish, too, and built like a tank.
It hasnt changed as much as people think. Stylistically it sure has, though.
There are more tall guys who actually arent lumbering idiots nowadays, but, even then, they are beatable by the 220lbr squad.
Tyson, Holyfield, Ali, Frazier, Wilder, Foreman, all could fight these ‘giants’, many of whom are carrying extra weight to be anything over 240 anyways.
id say boxing wasnt developed enough for Johnson to really hang today, but its less due to super evolution and more due to a sport being in its infancy.