Again, if the ball hits the sweet spot, the physics say it does not matter if Jon Brower Minnoch was holding the bat or the bat is not gripped at all. If it doesn't hit the sweet spot grip "strength" will play a part but player strength/mass will likely have minimal, if any, effects on the ball flight. What might happen however is with a stronger girl with slow bat speed she might be able to better continue her swing after the ball has long left the bat. Think about younger kids trying to hit those heavy TCB balls. Sometimes it appears as if their bat almost stops once they make contact. This might be mitigated some with a heavier/stronger kid vs a lighter/weaker kid both having the same bat speed. The ball flight would be the same in both cases however, assuming everything else is the same.