Tons of good food, tons of hard child through adulthood labour, plus weight training, and the fact that he probably weighed a bit less than your average man today.
Most of those guys that were ripped and muscular put in long, hard days at very physical jobs since they were boys, ate very little refined food, and were a bit on the lighter side. I can only think of a few that were over 200 pounds or close to it, almost all were #150 - 175 or so.