Which factor had the bigger part to play in the size increase of heavyweights during the 80s and 90s

Someone learned how to actually train taller dudes so they weren't just big oafs. Tubbs, Cooney, Spoon, Pinky, Dokes, Truth,. Buster, Ruddock, Tucker and Bowe and Lewis... all at least 6-3, most taller than that.

 

Of the ones you listed creation of Cruiserweight is the bigger factor. If you take the smaller guys out of the division now everyones training to fight bigger HWs and they are going to be carrying more weight.

Superheavyweights existed in large numbers going back to the 1910s. What you saw in the 70s and 80s was not HWs breaking new ground but a reversal of a 20 year slump with very few big HWs. Because there are no more combo fighters from LHW there will never be a slump of superheavys again because there can't be. If someone like Beterbiev ever goes on a run at HW 21st century boxings structure makes it so that can only happen one or two at a time at the top. 200s will never take over the division.

Part of weight increases was sporting societys attitude towards larger builds changed. While some HWs would carry 21st century weight your typical 6'3-6'6 HW favored a lanky build. That build is common outside of HW because its a means for tall people to sneak into the lower weight classes. But except Wilder HWs don't do that anymore.