That’s not the entire story. If you value your health over that of all others, you will want to crush the car the others are driving in over its full length before the crushing of the parts of your own car where you are sitting starts.

That’s where having the sturdier car can help.

You also will want to be in the car that decelerates less briskly. That’s where, in collisions with movable objects, having the heavier car helps.

A heavy car without crumple zones from the 1950s that could kill you when hitting a wall at 20 km/hour might still be the safer car in a head-on collision with a much smaller car with a few meters of crumple zone (cars from the 1950s were incredibly unsafe, so I think this would require extra-ordinary circumstances)

I'm trying to be careful how I type this, but the point of crumple zones isn't just to make the car stop slower, but to keep parts of the car you really don't want to crush from crushing.

If you crash into a wall at 20km/hr, it's a lot better if the engine bay gets smashed up, than if the compartment you're in collapses around you.

Given enough mass, you won't experience any noticeable deceleration during a collision! https://www.youtube.com/watch?v=hWToJxKYRSc