Why ah? Is it because they're heavier so inertia greater?
Most of the reason are due to inertia.
Inertia is the tendency of an object to remain in motion unless acted by an external forces.
the main factor affects inertia: MASS
The larger the mass, the harder it is to stop when driving at high speed.
Of course, momentum also plays an important role.
Since p = mv, huge mass means larger momentum, which could create huge force during collision.