They are taller and heavier. I don't know about the studies or numbers, but just imagine that you are walking, biking, or driving a small car, and you are hit b

They are taller and heavier. I don't know about the studies or numbers, but just imagine that you are walking, biking, or driving a small car, and you are hit by a small car vs. a truck or SUV. The location and force of impact is very different in all those cases.