What about limiting the number of sorties per some time interval based on the field size? Small fields can sortie, say for example, 10% of a side's population per 15 minutes, medium fields can sortie 25%, and large fields 50%.
That might limit player density, and it also makes some sense in a realistic sort of way - you just wouldn't see a large proportion of a side's players continually sortie from a small field.
Or maybe have each sortie realistic decrement the fuel/ammo available at a field, which would be replenished by truck convoy, trains, or air resupply missions? I'm sure HTC could experiment and find a reasonable sortie capacity rate for each size field. It might not end the hordes, but it would require them to issue from multiple fields and converge - giving the opposition time to find and intercept before the horde grows so large...
You could even tie the field label color intensity to indicate how much capacity is left at each field so pilots could decide where to lift off from a glance at the map, rather than jumping to a field tower and then finding the field hasn't enough petrol to fuel your bird for a sortie.
