The player height/weight data is taken from NFL.com player profiles.

That likely reflects increased specialization of body type by position, with the heaviest group containing offensive linemen and defensive tackles, the smallest group containing defensive backs, kickers, and some running backs, and the middle group containing other position groups. Nowadays if you're 6' 3" and 280 pounds, you're too big for most skill positions and too small to play line.

Americans have generally gotten taller and bigger over the course of the 20th century, but that's a complicated question. You'd also need to look at the growth of athletes in other sports over time. If all athletes have gotten taller and heavier at similar rates, then it would reflect a general trend, but if, say, tennis players have stayed roughly the same size while football players have grown a great deal, that would indicate that football is still an aberration.