Also consider that while a large truck does carry a significant amount of weight, they also distribute it over a significantly larger contact patch. While I will grant you that load on the asphalt is still higher than most cars, it's not nearly as straight forward as one might think. If someone with more time could google a comparison, that would be very enlightening.
Damage done to the road rises exponentially with the load. The rule of thumb is damage to the road is proportional to (gross weight / # axles)^4. A single fully loaded tractor trailer can do as much damage to a road as 1000 passenger cars. So I don't know if the higher fuel tax trucks pay completely offsets the additional wear they put on the roads.