[Demiurge]

It doesn't look like it. You can have different size airports serving vastly different volumes. It looks like the spacial distance is first factor, and then municipal boundaries. The boundaries are historical and can be based on population.

[jschulenklopper]

You misread the intention of the parent's comment. It's not that population _is_ taken into account (your interpretation), but that is _should be_ taken into account.

[Demiurge]

Hmm, could be, but how can you definitively determine that? :)