While football in the mid-80s could be pretty brutally violent, it's worth remembering that these players were, typically, a lot smaller than the NFL behemoths we're so used to today. The chart above—pulled from an interactive visualization created by Reddit user abresler—shows the average weight of each draft class for offensive linemen (black), defensive linemen (green), tight ends (blue), and linebackers (brown). The other positions saw comparatively little change in their weight, but you should go take a look at that data (plus info on height) on the original graphic.
The 34 offensive linemen drafted in 1974 weighed, on average, 255 pounds. By 1999, the 34 drafted offensive linemen weighed an average of 317 pounds, a 62-pound, 24 percent increase. During the same span, rookie defensive linemen gained 34 pounds, tight ends gained 33 pounds, and linebackers gained 19 pounds. These weights seem to have been pretty stable since then, which probably speaks more to the limits of human weight:speed ratios than it does to any sort of commitment by the NFL to its players' health.