1) More mass as mentioned in previous posts.
2) More actual space for additional steel and crumple zones. Imagine how far a Chevy Suburban type vehicle would have to crumpled in a rear-end collision in order for the damage to impact the second seat row passengers. Now compare that to a small economy hatchback. The full-size SUV literally has 5-6 feet of space between the seats and the rear bumper for example. The same applies to other outer dimensions as well, only to a lesser extent.
Digging up data was difficult due to the large volume available, but here are some referenced quotes all from different sources. I tried to find data that factored out variables such as driver age, speed, etc. and focused on occupant injury issues, but did include a table of raw statistics. If you can provide any data that smaller mass = improved occupant survivability I would like to see it.
a) "The effect of vehicle weight or mass on safety seems self-evident. As has been shown in a large number of studies, all else being equal, vehicle occupants in a crash are better off in a heavier than in a lighter vehicle."
b) "Larger, heavier vehicles generally afford more protection than smaller, lighter ones."
c) "The extra weight of the heavier vehicle reduces the risks for its occupants but also inflicts extra risks on the people in the lighter vehicle."
http://72.14.203.104/search?q=cache:1I4AFh5zC4YJ:www.iihs.org/sr/pdfs/sr3903.pdf Driver deaths per billion miles by vehicle size 1996-99 models during 1996-2000:
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