The players today are not that much bigger, on both lines they are fatter, and faster as well, because they train better and are in better shape for $$$$$$$$ reasons. Again, it's a rough sport, if the players can't take it find something else to do.
But I think you overestimate the difference in size, speed. How can you tell they are faster today? This is a serious question are the times listed somewhere of the older players in question?
Just a random selection, I'll leave it to you to look up when they played.
Richard Marvin Butkus
Position: LB
Height: 6-3 Weight: 245 lbs.
Jack Ham
Position: LB
Height: 6-1 Weight: 225 lbs.
Charley Britt
Position: DB
Height: 6-2 Weight: 180 lbs.
Ray Nitschke
Position: LB
Height: 6-3 Weight: 235 lbs.
Deacon Jones
Position: DE
Height: 6-5 Weight: 272 lbs.
Richard Paul Anderson
Position: DB
Height: 6-2 Weight: 196 lbs.
John Sawyer
Position: TE
Height: 6-2 Weight: 230 lbs.
Jim Brown
Position: FB
Height: 6-2 Weight: 232 lbs.
Joe Greene
Position: DT
Height: 6-4 Weight: 275 lbs.
Bibbles Bawel
Edward Raymond Bawel
Position: DB
Height: 6-1 Weight: 185 lbs.
Bart Starr
Position: QB
Height: 6-1 Weight: 197 lbs.
John Schuhmacher
Position: G-T
Height: 6-3 Weight: 271 lbs.
Reference
http://www.pro-football-reference.com/