[echoangle]

And can you explain why less momentum is better for the passengers? The thing that injures you in an accident is acceleration (or more accurately deceleration), so you actually want more momentum.

Imagine sitting in a large truck vs a kei car and driving into a brick wall. One has the chance to deform the wall which decreases the speed comparatively slowly. The other one abruptly stops when hitting the wall.

Unless the collision target doesn't deform at all, the higher momentum is better. And if the target is completely solid, a larger typically has a larger crumple zone which is better, too.

I cant come up with any reason to prefer the smaller car/momentum, except in very constructed scenarios (driving over a cliff with a weak fence for example).