American football is known for its hard hits on players, and as a result, injuries happen way too often. Players with more mass may not have a faster acceleration towards another player, but because of their mass they often times cause more damage. Picture a smaller running back running into an defensive lineman. The lineman don't have to move with nearly as much speed, simply because they are a lot bigger than the running backs. That is why the running backs are often the ones that fall backwards and on the ground because they have a smaller mass. They may have a faster acceleration, but the applied force of the defensive lineman on the running back is a lot greater than the applied force of the running back on the defensive lineman. That is physics right here.