I think limiting the size of large realms is riskly. Particularly a limit set at 25, which I believe would be too low. In most cases, there's a reason why the large realms are large. Players in them work hard to create welcoming atmospheres and help new arrivals - which is exactly the sort of environment you want new players to be entering to help with retention. The large realms are the realms that are doing something right. Blocking players out of them would be totally counter intuitive as they're the successful realms. In particular, forcing brand new players to join tiny realms that might be as dull as ditch water isn't a great idea.
Also, if you put a cap on the total number of nobles in a realm, before very long you'll see an epidemic of non-warrior classes disappearing. If you can only have 'X' number of nobles in your realm, warrior will become the essential class, because only warriors can secure the future of a realm. As far as I picked up from general chat, the introduction of the one character per island rule led to many people abandoning playing as courtiers, diplomats, and priests. Introducing another factor that encourages people even more to ditch these classes wouldn't be great.
The thing about the 'behemoths are negative for the game' argument is that Atamara is always held up as the disaster story. However, the Cagilan Empire has always been atypical. There have been many huge realms in the history of the game, and only the Cagilan Empire was really so dominant that it fatally damaged an island (although there is also an argument to be made that the Arcean Empire did the same in the Far East, there were exceptional circumstances around the Ice Age that influenced their dominance). This sort of behavior is the exception rather than the rule.
That being said, I wasn't ever advocating massive realms being created through untamed mergers. What I actually said is that if you think that around 25 to 30 nobles in a realm is a healthy number to try to encourage reasonable levels of interaction, every island can only really support in the region of 5 to 6 realms as a maximum. There just aren't enough players to support more realms than that and avoid having loads of realms with fewer than 15 nobles (which is apparently the level at which a realm is deemed too small to exist).
