There is something else to consider other than batspeed. This might actually explain the difference more than bat speed at the younger ages.
What I am talking about is player strength or their stiffness to the ground (balance, core strength, player size, etc). The bat is connected to the player who is connected to the ground through their body. The force delivered to the ball must react the force from the ball itself. If the connection between the bat and the ground is weak (as in a weak or low force spring), not all of the kinetic energy of the bat goes into the ball. I believe this is why we teach good balance, developing core strength, keeping our head down (to engage those special muscles in your back), etc. Do you agree Howard?). Therefore, the larger girl, even though she may be swinging the bat slower, can convert more of the kinetic energy into the ball since she is stronger and has more mass (essentially a stronger spring connection to the ground) than the smaller girl who may be whipping the bat at a faster speed.
I believe I have actually seen smaller girls bats appear to slow down on impact with the ball in slow motion video. (my daughter when she was young, for one.).
My guess is that this effect is a stronger influence when the girls are young. As they get stronger and improve their fundamentals, bat speed (and the mass of the bat) become much stronger influences on the distance the ball will go. I also think the physics of what is happening is more complicated than the way I am describing it, but the bottom line is size and strength of the player also influence the distance of the ball.
Kurt I pulled this off the net:
"In
classical mechanics the term
ground reaction force (GRF) refers generically to any force exerted by the ground on a body in contact with it.
[1] For example, a person standing on the ground exerts a force on it (equal to the person's
weight) and at the same time an equal and opposite ground reaction force is exerted by the ground on the person.
The use of the word
reaction derives from
Newton's third law, which essentially states that if a force, called
action, acts upon a body, then an equal and opposite force, called
reaction, must act upon another body.
The force exerted by the ground is commonly referred to as the reaction, although the distinction between action and reaction is completely arbitrary and the expression
ground action would be, in principle, equally acceptable."
I think you are correct in that we balance and sequentially load the core by bending at the waist first and softening the knees next and have great balance. We do not use a balance beam or 2 x 4 to hit off of so we need to be at heel plant to maximize the ground reaction forces you alluded to.
The next huge issue is when we hit the ball most girls stop their swing and you will see the bat go up against the shoulder or dump over their back between the shoulder and neck. By doing the hand over drill where we reverse the top hand upside down and pull the bat out of the top hand they get a better idea of making that good first move with the lead arm elbow and actually hit the ball harder with the lead arm verses both hands because they never get to extension. Then when they use both hands they get that big smile. We worked with Mel at Beaver Creek on this drill and she put the next pitch to right center at the base of the 300 foot fence...she is so much like her Mother and stays focused!