How is limiting kingdom size gonna affect multis in any way? They already don't need to be part of the original kingdom since no one gives a shit about trophies on a multi. It's actually prefered to have multis in separate kingdom for the sake of farming/chiefing.
Has nothing to do with multies. Its more about gameplay. Amount of treasures is generated based on number of players in kingdom and so we end up with huge metas. If kingdoms were limited to say, 80ppl, players would have to fight instead of uniting.