It's called physics

From a WSJ op-ed, on the greater death risk of being in a small vehicle in a crash:

The super-high efficiency minicar has become the Holy Grail for many environmentalists. But on Tuesday, a new study on minicar safety tossed some cold water on the dream. The Insurance Institute for Highway Safety (IIHS) reported that in a series of test crashes between minicars and midsize models, minis such as the Smart car provided significantly less protection for their passengers.

The tests did not involve the much ballyhooed mismatches between subcompacts and Hummers, but measured the effect of relatively modest differences in size and weight. Even though the Smart car and other minis such as the Honda Fit and the Toyota Yaris have fared relatively well in single-car crash tests, they performed poorly in these two-car frontal offset collisions. In the words of IIHS president Adrian Lund, "though much safer than they were a few years ago, minicars as a group do a comparatively poor job of protecting people in crashes, simply because they're smaller and lighter."

That difference is reflected in the real world. The death rate in minis in multi-vehicle crashes is almost twice as high as that of large cars. And in single-vehicle crashes, where there's no oversized second vehicle to blame, the difference is even greater: Passengers in minis suffered three times as many deaths as in large cars.

Given the nonstop pronouncements we've been hearing about the green promise of high-efficiency cars, these results were shocking to some. But not to IIHS. The Institute has long been reporting similar results from other tests, and its publications candidly advise that, when it comes to safety, larger and heavier cars are generally better.

That's not what advocates of higher fuel-economy standards want to hear. Greater weight may increase crashworthiness, but it also decreases miles per gallon, so there's an inevitable trade-off between safety and efficiency. A 2002 National Research Council study found that the federal Corporate Average Fuel Economy (CAFE) standards contributed to about 2,000 deaths per year through their restrictions on car size and weight. But amazingly, with the exception of IIHS, there's practically no one else providing information on the size-safety issue: ...

Read the op-ed here. I know, we probably ought to file this under "Well, duh." But the reality is that most people probably don't realize the decreased safety of small vehicles. Chalk that up to poor science education in the schools.