The difference is a lot of trucks are only allowed on designated truck routes as they damage roads. Even then the most destroyed roads in my city are the ones where these heavy vehicles literally work ruts into the asphalt. Not to mention its the state that maintains a lot of highways vs local government for local roads, and one is more fit to regularly repair roads than the other.

I imagine the pressure exerted by the smaller form factor vehicle will be significantly greater though...

On a pneumatic tire vehicle ground pressure is approximately tire pressure. The hummer will probably have some E-rated truck tire at like 50-60psi which is about what a panel van can be expected to have. Larger trucks are going to run higher pressures, 90-120 or so.

Highways are one thing, but local city streets/roads are not built to the same integrity.

18 wheels is the difference