You have mentioned previously that part of GRRM’s numbers problem is that Westeros is too large for a medieval setting. How were feudal kingdoms limited in their growth, and how large could they get before they ran into issues?

The basic problem was one of administrative capacity – feudal polities were incredibly dependent on the person of the monarch, and the monarch could only be in one place at a time and could only travel so far. And without a strong central bureaucracy, the monarch lacked any means of exerting control over far-flung territories that were nominally within their orbit.

So to give an example, the Angevin Empire extended from the Scottish border to the Spanish border, but it was very difficult to actually muster the resources of the whole nation, and as a result of the distances and lack of personnel the monarch was usually neglecting one part or another, which lead to rebellions in under-served territories and invasions by neighbors looking to gobble up some under-defended territory.