Sorry for posting again, but there seems to be some knowledge lacking about the subject. This time it's the transmission of the Prius.

"lot of the benefit is lost because you're carrying around a battery, a powerful electric motor and an extra bit of transmission to mix the power from each of the two sources"

The mechanical transmission of the Prius is the example of simplicity. It's only a planetary gearset, with fixed connections to the engine crankshaft and electric motors and a chain drive to the wheels. There is no clutch, no gearbox. This planetary gearset is about the size of a soda can. This is actually an area where the Prius is simpler and lighter than a conventional car.

According to my data, the total weight of the hybrid components is 171 kg. But you can not simply conclude that a non-hybrid Prius would be 171kg lighter. Without the hybrid components, the combustion engine would have to be heavier to compensate for the lack of power. A clutch and a gearbox would have to be added. The fuel tank would have to be larger, holding on average more kg's of gasoline. The brake disks would have to be larger due to the lack of electric braking, etc. etc.