I mean it's just momentum = mass*velocity. Something with more mass inherently has more momentum, and will therefore be harder to stop in an emergency/hit harder when it crashes. That's obviously more dangerous no matter how you look at it. A penny going at 60 MPH at your head probably won't kill you but a truck will. A penny going 1000 MPH will probably kill you, but I wouldn't call a penny as dangerous as a truck even though they're both potentially lethal.