On the big car VS small subject;
My for F150 4x4 will probably crush most small cars in a suitable accident. An F350 will probably crush my truck. A semi will probably crush an F350.
However;
In the last 35 years, I've never run into another car. I was rear-ended once while sitting at a light in a Dodge colt hatchback. I was the 4th car in the chain reaction, and we were not hurt. I only knew a hand full of people personally that were involved in multi-vehicle accidents. Some were fine, others died.
The point is that although the survivability is a concern, it's not #1 on my list. Accident avoidance is much more important to me. If I can avoid that swerving 18 wheel truck my chances of surviving go way, way up.
The smart might be OK for a second car, but I already have a Prius for general use and long trips, a 4x4 pickup for hauling stuff (a few hundred miles a year) and my wife's Camry (which is paid for) when we both need to drive at the same time.
I guess I'm still not buying one.