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.
