Sure adding that 2nd smart person, the 8th smart person, etc., can add benefit and yield a whole which is greater than the sum of the individual parts in isolation. But once you are in the organization size range where you're adding a 500th person, a 10,000th person, then the organization almost always takes on other qualities and conditions: bureaucracy, slowing down, middle management, rules, paperwork, gossip, insulation from consumers/competition, groupthink, aversion to risk, and so on. And these qualities dumb it down. The "group brain" becomes much dumber than a smart brain when left alone and unfettered as an individual.
Google is a bit of an exceptional case where despite having on the OOM of 20k employees they don't seem to have quite succumbed to Big Dumb Company-itis. Surely they have some aspects of it, but they've tried hard to fight it and stay like a smaller/younger org.
I once dealt with a startup that was around 12 people and already I could see symptoms of Big Dumb Company start to manifest themselves.