As I understand it weight, in and of itself, doesn't count for a whole lot and doesn't correlate directly to increased safety. It can help determine how much damage your vehicle will do but it has little influence on the safety of the occupants.

Semis and buses gain a benefit in passenger safety not so much because of their weight as the fact that the passenger compartment is well above the level where most other vehicles can strike it. If the passenger compartment is lower it tends to have to absorb more punishment.

Punishment a well engineered vehicle can better absorb without the passenger compartment being compromised. Volvo is a good example of a company that has put a lot of effort into protecting passengers. Innovations like designing the engine mounts and body so the engine will ride under the passenger compartment in a head-on collision.