So logistics.
There are many, many, many ways to house rule this. Obviously.
Here’s an idea though:
What if you imposed maximums on how many units can be in a given territory?
Say, 3-5 times a territory’s IPC value. Air bases allow 1-2 more air units, minor industrial complexes 1-2 more of any unit, and major industrial complexes 3-5 more units (undecided yet).
Territories with no IPC value can hold a maximum of 2-3 units.
So, what do you think? Perhaps it should be less strict, but this rule is meant to encourage a more even distribution of units on a frontline instead of consolidating units into one stack.
It also makes infantry less valuable. Not sure what the implications are.