Training to hit big can be achieved by using a combination of heavy and light bats.
Extra weight creates resistance in a swing, so greater effort is needed to swing fast, which builds strength and power.
Light, low-resistance bats are easy to swing fast and can be used to develop muscle memory.
This method of training improves bat speed which is one of the fundamental skills of power hitting.
NOTE: Do not use the heavy bat for too many consecutive drills because tired muscles could cause you to train yourself to swing too slow; and this obviously defeats the object of power hitting training!