You just gotta consider F = MA

The weight of you in your car doesn't change, so the amount of force you experience depends solely on the acceleration, aka the change in velocity.

If you're going 60 mph and crash into a wall and come to a very quick stop, that's one ouch unit.

If you're stationary and an extremely large vehicle going 60 mph crashes into you and you go from 0 to 60 mph, that's one ouch unit.

If you're going 60 mph and crash head on into an extremely large vehicle and you go from 60 to -60 mph, that's two ouch units. Double the force of the other collisions.

This is assuming that the extremely large vehicle doesn't slow down at all, which isn't the case in real collisions. If the large vehicle really kept the exact same velocity in the collision then essentially no force acts on it. Like a bus hitting a bug.