In the UK at least, Road Tax has no relation at all to the cost of repairing roads and is just treated as (yet another) tax.

And as a general point, any tax that is intended to encourage a behaviour should be revenue-neutral i.e. if you raise taxes for large cars, lower than for small cars so that the total tax take to government is the same. Otherwise it just becomes one more way for the government to ratchet up the amount of money they take from their citizens.