The laws of physics dictate that, all else being equal, larger and heavier vehicles are safer than smaller and lighter ones. In relation to their numbers on the road, small cars have more than twice as many occupant deaths each year as large cars. Large vehicles typically are heavy and small ones are light. But these two characteristics don't influence crashworthiness the same way.
Higher weight protects principally in two-vehicle crashes.
In a head-on crash, for example, the heavier vehicle pushes the lighter one backward decreasing the forces inside the heavy vehicle while increasing forces in the lighter one. This in part explains why when SUVs are in crashes with smaller passenger cars, the people in the passenger car are more likely to be injured or killed. This is the equivalent to a head-on collision between two identical vehicles, each moving at 35 mph. Instrumented dummies in the driver and front passenger seats register forces and impacts during the crash. In other words, the vehicle is being struck by a moving object.