That of course plays a role, too. But the idea, that automatically richer people drive bigger cars is not true. There is certainly a correlation, especially in the lower incomes, but this is only on a statistical basis, not on an individual basis. There are people for which a car has a high status symbol charakter, and those for which this is not the case. So some people buy big cars on credit and some richer buy smaller cars of their pocket.
So for making taxes according to what people can afford there are other places, like income tax or direct payments (or wealth tax or whatever you imagine), but taxation around driving should focus on the environmental impact only.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers