Question I couldn't answer about hybrids.

[particleman]

The Prius' engine is small and thus uses less fuel. Further efficiency is achieved with the electro-mechanical continuously variable transmission or CVT. If additional power is needed for things such as acceleration for passing, the electric motors will kick in. You would also get some gain from things such as coasting, braking, etc.

http://en.wikipedia.org/wiki/Hybrid_Synergy_Drive