Not all of them are. How about a 2L 4cyl putting out 300HP?
Too much? Use a smaller engine...How about a 500cc turbo, running at its optimal efficiency RPM, and running a big-@$$ alternator? When no juice is needed, it shuts itself off.
Add a couple (or four) deep discharge batteries, and the typical hybrid drive-motor set-up. The main differences being the smaller more powerful turbo engine, and the readily available car-parts store batteries.
I believe the big discovery in hybrid-car technology, is eliminating the transmission, and replacing it with an electric motor....NOT the rest of the hype.
The thermodynamic process of an internal combustion engine is notoriously inefficient ~ 20%. Diesel engines are slightly more efficient. And that doesn’t include the drive train.
I suspect that an electric motor is a lot more efficient.
Of course, when figuring efficiency a person has to include EVERYTHING!