CHECK OUT WHY FLYWHEEL IN ENGINES ARE TOOTHED !!

In general, power is only generated during power stroke while power is consumed during the rest three strokes (i.e. Suction stroke, Compression stroke and Exhaust stroke).A flywheel is large diameter metal disk attached to the crankshaft of the engine. High moment of inertia of this metal disk makes it difficult to rotate and stop (Newton's first law). It stores the power produced during power stroke and provides this energy during the power consuming strokes.

When electric starters came into the picture we needed to mesh the teeth of electric motor pinion with something to achieve high gear ratio for the required torque.That is where the starter motor engages with the engine to provide that initial torque. The teeth on the flywheel mesh with the pinion coming out of the starter motor.

It has nothing to do with weight reduction. In fact, the whole point of the flywheel is to have a rotor of higher mass to smoothen out the engine output power fluctuations.

Also See-CHECK OUT THE REASON FOR LARGER DISC BRAKE AT FRONT AND SMALLER AT REAR