In any conversation about team size, I think about this diagram:
A five-person team has 10 relationships and a nine-person team has 36 relationships. So adding four people to the team increases the number of relationships by 26, which is almost 4x.
The cost that usually gets talked about here is the communication overhead. More relationships means more time spent maintaining them and conveying information, which means less time “being productive.”
But the killer one is the hidden cost: 4x the relationships means 4x the drama. It’s 4x the opportunities for interpersonal problems, for personalities that don’t mesh, for misunderstandings, for unhealthy conflict, all that junk.
Scale carefully, and maybe stick the more difficult personalities on the smaller teams to shrink their blast radius.