The fact is that modern cars have astonishingly effective safety features that are likely to get you alive out of most crashes regardless of the size of your vehicle. The Insurance Institute for Highway Safety publishes data that shows that larger vehicles are safer but it is not like you die in the smaller car most of the time, but rather you are more likely to break your ankle or something.
If your vehicle goes under the tractor pulled by a semi (any size) or if it flips over the guardrail because it's too big to be held by the guardrail you do die.
My understanding is that safety ratings are relative to the class of car. So if you have a small car with excellent ratings and a large car that doesn't mean they will have the same survivability if they crash into each other.
The fact is that modern cars have astonishingly effective safety features that are likely to get you alive out of most crashes regardless of the size of your vehicle. The Insurance Institute for Highway Safety publishes data that shows that larger vehicles are safer but it is not like you die in the smaller car most of the time, but rather you are more likely to break your ankle or something.
If your vehicle goes under the tractor pulled by a semi (any size) or if it flips over the guardrail because it's too big to be held by the guardrail you do die.