In 2e, one of the main reasons domains didn't grow too much (more applicable to landed regents than non-landed as their expenses are much higher) was that the amount of holdings you had increased the cost of your domain maintenance. Simply put, the bigger your domain, the more costly it was to maintain it in addition to the stated examples of only having 3 actions. A quick and dirty way of going about this is by further reducing the number of court actions.

Take the base court (levels 0-10) and then multiply them by a number dependant on the size of the domain to determine court cost. So smaller domains have an advantage in being small to a certain extent. This further hinders regents that have grown too big for their own good. It is human nature for individuals to have ambition (particularly in players) to expand their holdings to beyond what they currently have. So the balancing act is to try to find that middle ground.

Take the guild situation in Ghoere. RoE states that Kalien has been able to outmaneuver HOT and kept his holdings in the west. This is because HOT has grown so big that he can't effectively face off against all his different competitors. Furthermore, maybe Gavin Tael has an interest in keeping Kalien around as he can always pressure the half-elf to reveal information from other realms in exchange for favours (like supporting trade routes through Ghoere) or getting him to act on his behalf in realms he doesn't have holdings by making it in Kalien's interest to help him for fear of getting his holdings removed. Also, Gavin might be concerned if HOT becomes too powerful and begins to have ambitions to remove him to place someone more sympathetic to his guild.

Another aspect to consider is that the landed regent does face competition from the non-landed regent. Maybe not directly in holdings but consider the consequences of military occupation or contesting such holdings. You're basically asking for retribution for such acts. Also, if you don't put up a fight, then you project yourself as weak and are thus encouraging other realms to target you in their own grab for power.

The other example is the Southern Coast. Roesone in particular has 3 different guilds plying trade in the barony. One of which nearly controls all mercantile affairs in Aerenwe. If the Baroness decides to promote OT as its pet guilder, then what prevents SG from going to Aerenwe and encouraging the kingdom to take umbrage to the baroness' preference? The same goes for Ilien and lesser extent Medoere. Diemed might be a different story of course, but then again he might be weary of allowing too much power to accumulate in a single guilder's hands to challenge his authority. Particularly when you consider that Kalien is a foreigner whose lands the Duke covets. Orthien Tane is a bandit lord and smuggler, and el-hadid is a foreigner whose interests lie in trade with Khinasi and he's also based out of Ilien; another land he may covet. Because these guilders also have interests in lands that are his traditional enemies, he may incur their wrath or make them stronger by forcing them to favor the guilds he is currently opposing at the expense of his pet guilder.