> Is it really a zero-sum game? If a small car crashes head on with a large car, will the people in the small car suffer worse than if they crashed into another small car? To what degree?

Well it's zero sum in the linear sense as the relative total velocity change is the same, but crash survivability isn't linear. Rather, it decreases exponentially, since kinetic energy is proportional to the square of velocity - so a head-on crash between a semi and a compact car subjects the occupants of the compact car to nearly 4x the force that they'd be subjected to if they crashed into another compact car.