If a heavy car collides with a light car head on at, say, 60mph, the heavy car might slow to 20mph whereas the light car might be pushed backwards at 20mph. So

If a heavy car collides with a light car head on at, say, 60mph, the heavy car might slow to 20mph whereas the light car might be pushed backwards at 20mph. So one car experience 40mph of velocity change in a short time interval, whereas the other experience 80mph of velocity change. This is hypothetical; I don't know what real numbers from real collisions are like, but light cars are definitely at a disadvantage in that they can get knocked around a lot more. There's also the possibility of a light car getting pinned between a heavy car and an immovable object or another heavy car. I don't think this is zero-sum: ideally, we'd all be driving around in large bubbles that weigh 200 pounds or less and have huge crush zones filled with styrofoam or something.