One of the critical tasks NFL scouts face in player assessment is evaluating players not just in a vacuum but also in relation to other players. Sometimes the task is to assess a player relative to a solid starter in the league, sometimes the task is to evaluate a prospect relative to the players at his position on your team's roster. Oftentimes, those tasks coincide.
Part of that evaluation is body type -- height, thickness, sturdiness, weight distribution, etc. That's one of the benefits of evaluating players in person, as area scouts do throughout the college season: They get a clearer sense of just how each prospect stacks up physically (beyond what is decipherable on film).
While working in the Chiefs' scouting department during the 2009 season, I recall seeing a player who -- even among his NFL brethren -- was nearly superhuman in stature. It was Vikings running back Adrian Peterson, who looked more like a linebacker than a ball carrier. It crystallized for me that comparing other running backs to Peterson was shortsighted: He is far from a prototypical running back.