Wouldn't an additional advantage of series hybrids be that the engine can be tuned to operate solely in it's most efficient RPM band, since it just charging the battery and doesn't need to deal with changing speeds? This can (according to some very cursory googling), result in efficiency gains of 20-30% relative to the least efficient RPMs. This should at least partially offset some of the size and weight considerations, since you don't need to size the engine for it's energy output in less efficient speeds. This seems like it would be most important in stop-and-go conditions where the engine is spending considerable time at less efficient speeds, such as in a bus.
The parallel hybrids can already do that because of the CVT.
In practice they also allow the engine to run at higher speeds under heavy acceleration because the peak efficiency RPM and the peak power RPM are different and the assumption is that if the driver is stomping on the accelerator they want to resolve the power/efficiency trade off in favor of power right now, which is another thing the series hybrids can't do.
