If someone crashes into you with a heavier car, then that's the responsibility of the owner of the heavy car - just like if you knock over a cyclist (or it should be). The ideal would be everyone driving slower in smaller cars, since reducing kinetic energy saves lives. To achieve that you reduce taxes on the smaller cars and increase taxes on the heavier cars and cars that drive at higher speeds. And change the law so if you kill someone with your big, heavy car, you get a more serious punishment.
It's the end of days for human-driven cars anyway. With self driving cars, it seems as if the cars will be bought and owned mainly by large companies (and hired out to end users), so the companies will be able to experiment much more aggressively with different designs and force manufacturers to make them.