Two things: Some hybrids (the
Two things:
Some hybrids (the ones that use the Power Split Device hybrid system) are actually mechanically SIMPLER than a normal car, not more complex. The Toyota Prius uses the PSD and it has all of 27 moving parts in its transmission, versus 100+ in a normal 6-speed automatic tranny. The complexity is in the control electronics, not in the mechanicals.
There are also efforts to decrease the weight of a hybrid system by moving to lighter lithium-ion batteries (versus heavier NiMH which was the battery of choice for the early hybrids). And Toyota has been incrementally shrinking the size and weight of the traction motor-- The MG2 in the 3rd-generation Prius is about half the size of the one in the iconic 2nd-generation Prius.
That said, I agree with Rob that the improvements made to a gasoline engine can be applied to a gas-electric hybrid to further improve fuel economy, since a gas-electric hybrid still has a gasoline engine aboard.