A vassal-like relationship works on the imbalance of power. You cannot have an imbalance of power when you artificially limit the size of realms. The vassal realm won't have to hit the limit to become strong enough to defy the parent realm. If the limit is 10 regions, then even a 7 region realm will probably be strong enough to tell the parent realm to go take a hike.
A 10-region-realm can perfectly oppress several other 7-region-realms with diplomatic skills and differential vassal-tributes between them, specially if it forms an inter-realm federation and whatnot, being more the
primus inter pares than the absolute monarch (in relation to the rulers of the vassal states).
Having 27-region-realms is pretty insane and unless strife happens from within them it completely stagnates the region.
