You have to remember by 1988, the sanctioning bodies had reduced all championships bouts from 12 rounds to 15 rounds.
So after that, there was a little more emphasis on mass.strength and power,at the expense of endurance. I'm not sure that it made heavyweight boxing better,though.
Also,heavyweight like bodybuilders ,on the whole have gotten distinctly bigger,since the early 90's .And like bodybuilders,the athletes presumably would have access to the latest pharmaceuticals.