[dpeck]

Middle group is likely to be primarily made up of linebackers, running backs, with a few of the larger tight ends there as well.

These players, while having some responsibility in the passing game are primarily involved in either running the ball, or defending against the run. And having a bit more mass there is very beneficial, but they still need to be able to have the speed to be able to work horizontal across the field and sometimes be quite a ways away from the line of scrimmage by the end of the play.

[maxerickson]

incision's link, in a top level comment, has a plot that distinguishes positions. (Modern) Tight ends are heavier than linebackers, then quarterbacks, then running backs.