Its based on actual deaths. It's more likely that more conservative (meaning... safer) drivers will buy the Corolla over the Prizm.
People willing to take a risk on "domestics" may just be more likely to take risks on the road.
Or that's how I read it.
Minivans, on the other hand, are driven by distracted moms... who never reach speeds that threaten their own safety, but sure as hell don't have any compassion for any bikers they run over.
-
Weight means little for crash safety. What matters is crash structure. And you're always best off with a midsized car with a four-banger or a front-wheel drive minivan. Least weight for the most crash structure. And that chart shows it.