Why would an NFL linebacker have a lot of momentum?

The bigger and heavier a player is, the more mass he has and the greater the force that's required to break his inertia. This is why a 200-pound running back will look to make quick cuts to the left or right to evade a 250-pound tackler-the running back has less mass and can more easily change direction than his more massive opponent. On the other side of the ball, that more massive tackler looks to use his greater mass to generate enough force to stop his opponent's forward inertia.