Not advocating one way or the other, but as an engineer with experience designing engines I know that a small engine running at 100% throttle will be more efficient than a larger one running at 60-70% throttle producing the same horsepower due to pumping losses. Some modern automobiles mitigate this with infinitely variable valve lift. A hybrid aircraft should be able to run a smaller engine in cruise and use the electric engine to supplement the power at takeoff / climb.
You can see similar benefits in hybrid automobiles. Although the massive improvement is in the city-cycle fuel economy, the highway economy of hybrid automobiles is also better than the highway economy of the equivalent non-hybrid version. Highway driving at constant speed is similar to cruise in an airplane.