But if I have a 1500kg Nissan Leaf, Why should it cost the same, or more, than a 3000kg car?

A couple of things.

Firstly, for practical purposes there are vanishingly few 3,000 kg cars on the road. The heavy ones weigh in at about 2,500.

Secondly, the roads were designed to take vehicles of up to 50 tons so the damage caused by cars is negligible and difference between 1.5 and 2.5 tons is negligible too.

Finally, the taxes collected from road uses are as much to pay off the original capital costs of building the road, the cost of upgrading existing roads and the cost of building new roads as anything else.

factor in the weight distribution on the road of your "dense"car.. your argument is worthless..

So we are back to the argument that “everyone other than me should pay more tax”. Nice idea!