In general: whoever's in the smaller car, comes off a whole lot worse. Driving something the size of a tank is good for you.

And bad for everybody else. Total utility is probably negative because the process where people drive larger and larger cars in order to be safer (at the expense of others) leaves everyone roughly equally safe, but driving more expensive, more fuel-hungry, less-manoeuvrable cars.

Whether this is reason not to buy a larger car depends on one's level of altruism, of course.