Of course it's possible. X members per kingdom at max; you can only reinforce players in your own kingdom. Bam. Done. Creating multiple wing kingdoms just lost its purpose.
"its not possible" as in "its not possible to make it practical and fun"
In the situation you just described people will do what is happening on large Legends servers right now. People will get rotated. You just need one village in each kingdom borders, which isn't that hard. Deffers will get invited to the main kingdom whenever they are needed. Off players will only go to the main if they are participating in an op.
A mechanic that promotes small kingdoms can be gamed by creating a meta alliance where the "main" kingdom is small. It's literally the same "exploit" you just said people would do if large kingdoms were punished
Obviously not in a way that can be meta played you absolute buffoon. I'm not talking about a morale buff to small (pop) kingdoms that then amass a ton of treasures. The morale would not apply to anyone who is ranked higher than you in pop, vp, or current treasury count or up to 20% lower.
Still, a stat buff (like morale) for small kingdoms is not great. But there for sure need to be more anti bully mechanics. Extra cranny capacity still sounds really good to me after I've seen how well it worked in Legends.
Regardless, are you going to contribute any ideas of your own or are you just gonna try to shoot down others with logical fallacies?