What's interesting is before the 1970's it appears as though there is just one group of players, at which point they become three. Then, over the following 40 years these groups seems to just get heavier - most likely a response by teams to maintain heavier players than their opponents in certain positions.
While the general growth over time makes sense, I wonder what caused the shift specifically in the 70's.
So, weight training and anabolic steroids really take off in this time period. The added strength probably means players can still move that massive body weight at the same speeds (or, in most cases, much, much faster) than someone who doesn't add bulk muscle as modern NFL players do.
NFL players are absolutely, positively freakishly fast for their size.
While the general growth over time makes sense, I wonder what caused the shift specifically in the 70's.