To bring a little different perspective on this, here's a guy who prompted a trend "behind the scene", without really having that change translate to what was necessarily rewarded onstage.
Pete Grymkowski (Front of the line with the 'stache. In 1978, weighing 250) was one of the first guys to push the envelope on his anabolic intake, in the early-to-mid '70s, taking 3 to 10 grams of total anabolics per day at some points. I don't know much about steroids, but I've been told this is more than some guys now take in a week.
As a result, he was one of the first guys to be on-stage in contest condition in the 245-250 range, at 5'10". On the flip side, he won his weight class in the Mr. America (lost the overall) and won the Mr. World, but never placed higher than 3rd in the few other pro contests he entered. Funny/interestingly enough, he doesn't seem to have ever competed in the Olympia but I haven't read a reason why.
With that in mind and without getting too psychological, maybe he was like Roger Bannister breaking the 4-minute mile. Other guys saw Grymko push the anabolics and contest weight, so something clicked like, "Ahh, I guess we could do some more", and that started the snowball that rolled into the '80s.