My assumption is that the world exists as it does for reasons (rather by accident).
[ul]
[li] realms are the size they are for a reason
[/li][li] realms and domains are not easily toppled, many have been around a very long time
[/li][li] war is pretty frequent, but it doesn’t much change the map
[/li][/ul]
With these in mind, I outline what war looks like. There is a great deal of advantage to the defensive, so that offensive war is challenging. If the advantage lays with the defender, it makes sense that there are so many small states. All they need are the fundamentals of defense. Part of this is a good network of castles. Part is reflected in the difficulty in raising truly offensive units. Part is reflected in the way militia works.
Fortifications
Given the age of the setting and its reasonably static nature, I would imagine that there are many very strong fortresses whose capture would be long and difficult and would represent a significant event. One such example of what this might entail would be the historical siege of Calais, which lasted from the Sept 4, 1346 to Aug 3, 1347. Seven to Eight thousand citizens held off 20,000 Archers, 6600 English Infantry, 2000 Flemish Infantry, and 5300 Knights. In this example, the French never tried to lift the siege. French supplies arrived by sea, but the English tried to stop them and eventually set up an effective blockade.
Imagine the difficulty in taking Ilien if the same circumstances were in play. It would not be hard to imagine the same overall situation, after reading a description of the events surrounding Calais. Despite the fact that 34,000 man armies would be exceptional in BR (what is that - 170 units sitting around waiting for surrender?) there is no real mechanism for a siege to last that long.
What if the way Roesone, Ilien, and Medoere gained independence was that the founder-heros gained control of key fortresses by treachery, the citizens throwing open the gates, or other means? Then Diemed spent the rest of the civil war winning battles in a futile attempt to recover the lands lost, because battles won mean nothing without capturing the key fortresses in the realm.
Elite Units
Perhaps the reason realms maintain the forces that they do is that they cannot raise units of such quality when they need them, so they keep them around so they have them when they need them. As such, additional troops raised would be no better than irregulars or levies. Growing similar units would either be slow, or would require experience.
Levies
There might be levies who rise up for the defense of the province and disappear in all other circumstances. This would also significantly improve the defensive posture of realms without increasing offensive capacity. Levies could be smart, too. They might bide their time until the defender’s main army entered the province, rather than rising up to get defeated by an unopposed attacker. Instead they might engage in Robin Hood type guerrilla warfare.
Another thing that would tend to maintain the staus quo is a dynastic tie to the land. Even when some new person gets invested in a province (or even a whole realm) the old tie isn’t broken, and can be easily re-established as soon as you occupy the province.