Who is the target audience? Insurance companies already have the statistics. Buyers care about occupant safety and would largely ignore a pedestrian safety rating.
If you tried to force it into a combined rating you'd probably make the problem worse because then people would know that large vehicles are being punished in safety ratings and refuse to buy small vehicles even more than they do now because they can't distinguish whether a good safety rating is from occupant or pedestrian safety.
> people currently buying unnecessarily large vehicles wouldn't have a lot of overlap
Maybe I'm naive, but I think most people buying large vehicles aren't selfish, they're ill-informed and susceptible to social pressure and advertising.
The only reason I say this is because most people have big cars in the US now. But they're also objectively worse for most commuters. It doesn't add up.
If you tried to force it into a combined rating you'd probably make the problem worse because then people would know that large vehicles are being punished in safety ratings and refuse to buy small vehicles even more than they do now because they can't distinguish whether a good safety rating is from occupant or pedestrian safety.