We already have something that tells us which duchies need knights, and "glory" which vaguely translates to how many battles in its regions. Of course these are all really vague anyway.
But making it more exact, that makes me wonder if the bigger realms might not just get bigger, and by extension, have the knights to keep growing bigger, while the smaller realms don't change much. The reason I say this is because I think some of you might have the wrong idea about the typical player who joins.
The typical player most likely has not the slightest clue about positions. Probably doesn't even know they exist, or what's so special about them. When they look at where they want to start, they're not thinking about getting a position, at least when I started I didn't know or care about positions. When my roommate in London wanted to start playing, he asked me which place had the best tactics in battle. The first question wasn't which realm had the best social mobility. It was about the action.
Now, some people might in fact go in knowing about all the positions, and be ambitious. But I think that for the vast majority of true new players who know nothing more than what was advertised, they join for the battles. So, a big realm generally has a stronger military. And I think most people who are naive to the inner workings of any environment would prefer a larger place. That's the same reason why in other MMOs the largest most prestigious guilds have the most applicants while the smaller guilds might at most have one or two curious or seriously interested individuals.
Also there is the factor of risk. It's considerably riskier as a new player to join with a small realm than a large realm because the assumption is that the large realm is well entrenched in its position. The smaller realm on the other hand, might be unstable. While this might not necessarily be true, that, I believe, is the common outside perception of those newly arrived to this game.