It can't all be about a change and availability in drugs, surely? Back in Arnold's day - and before - the general consensus is that steroids were purer, more easily available (i.e., legal) and cheap.
If you look at that recent Photoshopped picture of Arnold and Ronnie together, their upper mass - from the front at least - is pretty comparable. Yes, Ronnie has better separation, lower BF etc, but there's not an enormous difference in the mass stakes. Their legs, however, are a world apart - Ronnie's look almost twice the size.
What actually changed after, say, the mid-1980s, that saw legs really start to grow in the majority of pros? Judges started actually looking at them?
I can't quite see that drugs have made any difference, so was it something like the availability of the leg press/hack squat machines etc that let to pros being really able to add weight and thus bulk to their quads, or did pros back in the day simply not pay as much attention to legs, and it was only uber-trainers like Platz who managed to take them to the next level, probably because he was doing as much/more work than the modern pro?