He's a lineman. His role involves being a literal wall to protect the quarterback and/or running back depending on the play. Size helps, if nothing else just because that makes it more difficult for a defender to get around them. The fact that linemen are only generally relevant for about 7 seconds each play enables them to be able to sprint full speed, delivering huge amounts of force if they make contact with a defender on a run play. I'd say it's a decent amount of muscle, not just fat. These guys can bench 225 lbs ~(100kg) for 20-30 reps coming out of college. That's insane. They are tanks in human form