It's an arms race.
I used to live near a couple who were both doctors in the ER and they both drive the biggest trucks that they could find because they saw that people in large trucks tended to be fare better in accidents.
I have a small sedan for myself and a smallish SUV for my wife and kids. I feel pressure to upgrade both to something larger.
Larger potentially means even in a single-vehicle accident, there is a greater distance to decelerate over and things are less likely to intrude into the vehicle.
The IIHS, which systematically tests vehicles in simulated crashes, says:
> A bigger, heavier vehicle provides better crash protection than a smaller, lighter one, assuming no other differences. The part of the vehicle between the front bumper and the occupant compartment absorbs energy from crashes by crumpling. As a result, the longer front ends of larger vehicles offer better protection in frontal crashes. Heavier vehicles also tend to continue moving forward in crashes with lighter vehicles and other obstacles, so the people inside them are subject to less force.
https://www.iihs.org/topics/vehicle-size-and-weight
There is some simple actuarial data there on their page, too, which shows there's a marked advantage but it is less than it used to be.