A bat that is TOO HEAVY will slow down your swing and make it very difficult to maintain proper balance during your swing. Starting with improper balance only leads to improper form in swinging, which reduces the chances for a well-hit ball.
A bat that is TOO LIGHT will cause you to swing too fast, which also affects your rhythm and keeps you from driving the ball. A light bat also sometimes causes your shoulders to get ahead of your hips.
In an EFFECTIVE SWING, the hips should lead the rest of your body. Most of your power comes from the action of your hips!