As noted earlier, numbers I provided are meant to demonstrate how this could work and not meant as real in-game numbers. Additionally, I have no thoughts on whether linear scale or otherwise would be best for scaling numbers.
It may make sense to extend extra XP sources to other passengers, like gunners, commanders, and copilots.
On the logistics side of transportation, it might make sense to give XP to more than just those unloading the supplies, but that may be trickier to design a good solution.
A possible solution to avoid abuse is to split XP between players in (some of?) these situations, instead of increasing the amount of XP that is granted (by giving more to the driver than they previously would have gained).