I think the major difference in physiques today versus those of the 80s and early 90s is their training styles. Back then, the name of the game was high volume, high reps, and moderate weight. A lot of the smaller more symetrical bb were just pumpers by todays standards.
As Bob Chick pointed out on bb radio, it was Dorian Yates that changed bodybuilding forever. Dorian's training style popularized the notion of heavy max weights with lower reps and fewer sets. Now before you chalk this up to Mentzer, consider also Dorian's own unique invention - the 70 degree underhand grip barbell row. If you watch older videos of Haney and Gaspari and look at pictures of Arnold doing t-bar, they performed these exercises with their body parralel to the floor. They were so hunched over that they could only put 25's on the bar because 45's would hit their chest. By modifying the style of back training, heavy weights were possible and more muscle was built. Consider the way Ronnie does t-bar rows. He's pretty much standing straight up and can lift a shit-load of weight (600+ i believe).
In even more recent times, the increase in size is due in large part to a resurgence of powerlifting movements. Today, far more pros are doing deadlifts and low rep sets. Drugs must play some role, but training style and the complete reinventing of back training by Dorian Yates had a large impact.
