For kingdoms it’s kind of a goal to expand and have more members, the whole game is design around it.
The thing is this expansions shouldn’t be friendly like they usually are, they should be done under intense war, destroying, conquering and gaining territory step by step.
Limiting kingdom members would go against everything kingdom developers have been doing, and stil favour the "metas". I am confident to say it won’t even be tested.
I agree with past comments, the problem is in the players, Limiting members would just mean much more food for Metas xD