Apart from 'road tax' isnt road tax, its vehicle excise duty, based on emissions. Really i guess it should be a combination of emissions and mileage to be truly fair, however on the whole, big cars tend to do more mileage and be less efficient and thus the tax works if the price of the tax bands takes this into account.

VED doesn't pay for the roads, Council tax pays for wear and tear on the majority of local roads, VED just goes into the main pot.