Since the heavier vehicle has higher momentum, it would decelerate slower when hitting a semi-static-object, which decreases the forces on your body. The smaller vehicle would stop much quicker and you would jerk forward. So a heavier vehicle at the same speed would probably be better, all else being equal. And that's neglecting the additional material a larger vehicle typically has that deforms over longer distance than with a shorter car.
So many are and some aren't. So in many cases it doesn't matter which car you're in, and in some cases the heavy car is better. That means the heavy car is still net better.
I still haven't seen a single argument why a smaller car would be better in a collision.
But a small car doesn’t perform better, why would it? Surely a large car is better when driving into an electrical pole than a small car. What advantage does the small car have?
Also, just to clarify, how did I change the problem? The problem with car crashes is rapid deceleration which injures you, and lighter cars are worse in that aspect compared to heavier cars.
Crumple zones have been a thing for almost a century.
Momentum doesn't matter. It matters in a vehicle on vehicle collision, but since the pole is essentially infinite I'm not sure why you're bringing it up. Acceleration is what impacts the human body, and it's determined by speed and distance. For the same speed, a larger car will have a longer distance to stop, therefore lower acceleration, and less force on the body.
Even if you assume that the pole doesn't move at all, having less momentum doesn't help you. A smaller car isn't better, in the best case it is equal. But a larger car probably also has a larger crumple zone so it is still better. You still haven't given a single reason why a smaller car would have an advantage.