Question: why are airport landing fees determined on a per-passenger and not a per-airplane basis? Who determines this? Is it something that each airport independently came up with, or is it something pushed down on them by the FAA or some other federal agency?

I ask because it is clear that one of the consequences of this pricing scheme is to push the airlines towards flying more smaller planes instead of fewer bigger ones, since it is one, often non-trivial, component of the cost that scales with passengers, not planes.

Thanks,
saunders111