Actually, single vehicle crashes are the largest single type of crash in the US. In my region 46-52% of car crashes in the past 5 years were single vehicle crashes. Of those, the vast majority (over 2/3) involves the vehicle hitting a fixed object.
But in general, you are correct - all things being equal in a crash, being in the heavier vehicle is an advantage.
You also need to acknowledge that, all things being equal prior to an accident, the lighter vehicle has a better chance of a voiding it.
So pick your poison.