I think this is going to be the natural progression of hybrids for this reason:

Engines tend to perform best right in the middle of their output -- not too low, not too high, without a lot of load changes. Smaller engines will generally be more efficient. Elecrtic motors tend to perform best at a small fraction of their peak output (reduced internal elecrtical resistance), so a much more powerful motor will generally be more efficient. Combine a high peak power motor with a small, efficient motor and you have supercar performance with hypermiler fuel economy. Unlike pure gasoline cars, having all that excess horsepower should actually help overall efficiency.

Parallel hybrid designs (both the engine and motors power the wheels) seem to be preferred right now as the engine power can be added to the elecrtic motor power, and under steady state cruising it's not necessary to transmit any power via electrics. The series hybrid designs will likely take up more space (motors big enough to power the car AND a generator AND and engine) as well, but the main reason we don't see them is probably cost (much more robust batteries/motors/elecrtonics will be needed).