The larger a team’s size, the more it needs to communicate.
When you scale up a team from a size of 6 developers to 12 developers, how much more communication work does this entail? On the surface, we are led to think that we need to communicate twice as much. However, 6 developers can communicate two at a time in 21 ways (6 x 7 / 2). If you scaled this team to 12 developers, that number shoots up to 78 (12 x 13 / 2). That is 6 times more communication channels than the smaller team.
When we double a team’s size, we expect productivity to double as well. However, given that communication work need by 6 times, the productivity gain is a lot slower. This led Fred Brooks to coin Brook’s law – adding manpower to a late software project makes it later.