I'll point you to my previous post. Why do you assume that those players from previous eras would be that small in today's era? Do you think genetics have vastly improved over the past 50 years? It takes thousands of years for genetics to markedly show change over a species. Why do you think JJ Watt would be 6'6" 290 lbs of chiseled muscle in the 1950s? That didn't exist. Sure, Watt is an outlier, but in NFL circles, he's not that much of an outlier in terms of size and strength. Mario Williams was about the same size when he was here.
Truth is, things have changed in the way elite athletes prepare, train, recover, rehab, eat, and study. Even if you set aside the prevalence of hGH, you still have way better nutrition and more importantly, nutritional information. Most of the guys you see in the NFL are tuned into that as young teens. They and/or their parents are making sure they eat right, eat a ton of calories, and work out. Kinesiology has gotten better. Athletes now train and recover with the type of focus that results in what you see out there. It's no secret or surprise that NFL players shrink (or die) once they stop playing. It's not natural for the human body to be like that. I guarantee you once Watt stops playing, he won't be 290 lbs of chiseled muscle. If he's 290, it won't look good.
Those players from bygone eras would have access to all the same nutrition, technology, techniques, supplements, and training staff that Watt has access to. It's silly to think they'd be 40 lbs smaller than all their contemporaries.