Where is the data from?

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

What's up with the blob separating into multiple groups in the 1990s?

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.

Didn't all Americans get bigger over this same time period? Is this just a proxy for that?