I worry that this could be a bit exploitable if it also applies to minors/free folk. On some worlds you can get 60,000+ Free Folk Marauders in a big zone, which you can easily crush one at a time with a single brigade, suffering few losses and forcing each unit to surrender by encircling them.
I'm skeptical of this claim. A subunit is 100 infantry, so that's 600 sub-units in a single zone.
In any case, there would need to be scaled penalties or costs that would make absorbing such a large population undesirable. Unrest, money, pop happiness, production penalties, something like that.
Oh right, blep, my mistake. I was remembering 38 stacks I saw and my brain automatically translated it into thousands. Blame the unusually small scale of the game :P