We already have a low enough density that grabbing land is not hard. If we make it any easier we will remove any incentive for war and we will be left with realms with no enemies and no reasons to make any.
I disagree with your notion that the only reason for war is establishing control over rural regions. I further disagree that if realms are discouraged from biting more than they can stomach they will stop having enemies and conflicts. Conversely, being unable to control as many regions as they want, underpopulated realms will actually be incentivised to fight for the richest ones just to keep their economy stable.
Lastly, the only way to create a realm or destroy one is to grab cities and that's never easy. So there will actually be no change in that regard.
