Comment by dghlsakjg

Possibly in some designs. It comes down to a weight issue. The entire weight of the hybrid system needs to be less than the weight saved by installing a smaller ICE engine.

Take a modern small aero engine like the Rotax 912. It comes in an 80hp version that weighs 122 pounds and a 100 hp version that weighs 144 lbs. Hard to beat that kind of power/weight.