You are transferring rotational energy to the ball. Rotational energy is proportional to square of angular velocity. It is only linearly proportional to moment

You are transferring rotational energy to the ball. Rotational energy is proportional to square of angular velocity. It is only linearly proportional to moment of inertia. So bat speed is more important than weight, all things considered. From what I can tell this is more about making it easier to make solid contact. Even if it lowers the moment of inertia, it’s more important that the ball goes on a decent trajectory rather than e.g. directly into the ground. A bigger radius means the ball will be exit closer to the plane of the swing.