I find it hard to believe that the mass of an individual has anywhere near as much to do with how far they hit a baseball as strength does. Take a baseball player, and have them eat 15,000 calories a day until they add on 100 lbs of fat. That baseball player is now going to mass more than they used to, correct? Are they going to hit a baseball any further? Hell no. There is no reason why they would, and several reasons why they wouldn't.
Therefore, it cannot be a simple case of mass of the batter that has such an impact. Otherwise wearing lead underwear would make someone a better hitter, because their overall mass would be more than it would if they wore ordinary boxer shorts.
-Arlos