The lower risk subcompacts were found in real world crash statistics to have LOWER FATALITY RATES THAN TRUCKS OF ALL SIZES, up to and including the largest category of "passenger" truck, the 1-ton; which, despite the name, weigh in the range of 3-4 tons, up to 4 times as much as the sub-compacts that are safer than them.
"All other things being equal, occupants in a bigger, heavier vehicle are better protected than those in a smaller, lighter vehicle."
That sentence stands alone, as though it were universally factually accurate... but then soon after it is qualified by the important distinction that makes it accurate:
"Weight comes into play in a collision involving two vehicles. The bigger vehicle will push the lighter one backward during the impact. As a result, there will be less force on the occupants of the heavier vehicle and more on the people in the lighter vehicle."
(Then, to demonstrate this, they have a graph not of vehicle weight, but of vehicle size relative to risk (the bigger in size, the bigger the crumple zone))
"Unlike frontal crash test ratings, side ratings can be compared across vehicle type and weight categories. This is because the kinetic energy involved in the side test depends on the weight and speed of the moving barrier, which are the same in every test."
KE=½ mass X velocity²
Kinetic Energy= (1/2 of mass) X (Speed squared)
5 steel balls on strings, the first one is given a swing, and when it hits the rest, the force travels right though them to the last one. The last one takes just as much impact as it would if the first hit it directly, because the others are solid, and the force just goes right through them. It doesn't even matter that the 3 in the center have a combined mass of 3 times as much as the two on the ends. The one on the end is in no way "protected" by them, as they don't "absorb" any of the force, they simply transmit it.
This is why modern cars have crumple zones. They are deliberately, by design, weaker than the solid steel tanks of the past. Of course they aren't arbitrarily weaker - the human containing cabin is made stiff, while the front and rear are made soft on purpose so they take the impact energy.
Ask yourself: Which would you rather do, crash and survive, or not crash in the first place?
In addition to all those factors making them capable of avoiding accidents better, the lack of (false) perception of safety may encourage drivers of small cars to take fewer stupid risks (which are, ultimately, the cause of almost all accidents). The very fact that people feel safe in big vehicles make them do more stupid stuff, like speeding and reckless driving, than the drivers of smaller vehicles. It's called risk compensation - and its counter-productive when the assessment of risk is completely wrong.
And don't let the myth influence your next car purchase.