The horrible truth: productivity is inversely proportional to team size.
--Peckish's First Law of Software Engineering
I think it has something to do with communication overhead. It may be most pronounced at 10**2. After that, inter-group firewalls intervene… To save Peckish's Law, you need to define team size as the actually number of people with whom you must coordinate/communicate.
Communication and decision-making dominate, I'm afraid. Negotiation rules the day. As team size increases, it becomes increasingly more useful not that you do the hard stuff cleverly, but that you do the easy stuff adequately/reliably…
My favorite team size is 10**0. Beyond that, it's all politics
--Thomas Jay Peckish II, on Team Size...