That's not completely true, Eddie. A lighter vehicle can be just as safe (even safer) than a heavier vehicle. How heavy are cars today compared to the big boats of 40 years ago. Have you seen the Impala vs. Impala crash video? It shows the difference of an old Impala ('69 maybe?) to a newer 2000's model. There is no comparison at all, and I know which vehicle I would rather be in.
Sure, compare a 2k lb car and a 5k lb truck hitting a semi, and there will be a huge difference. Crash each of those vehicles into a brick wall, and the lighter one can be just as safe as the heavier one. Keep in mind, a heavier vehicle has more mass and much more inertia, so there has to be more crumpling of the heavier vehicle to distribute the forces of the crash.
Here are a couple links to the crash video I mentioned (and sorry, it was 2009 Malibu vs 1959 Bel Air):
Crash test Chevrolet 1959 vs Chevrolet 2009 - YouTube
Crash test 1959 Chevy Bel Air collision with Chevy Malibu Car Crashes - YouTube