As of April 2020, there are 124 staff and three layers - director, deputy director, and program managers. That number is right around Dunbar’s Number. It’s not explicitly designed that way but I suspect that it emerged as the right number for the Director to be able to vaguely pay attention to everything that is going on and for everybody in the organization to have vague context on what everybody else is working on. Institutions shape how individuals interact.
There are three layers - the Director, office directors, and program managers (with a few people like deputy director thrown in.) Culturally and physically DARPA is set up so everybody is talking to each other all the time, so there is lots of ability for Idea sex.
The size of DARPA may help minimize internal politics. The returns to politics is higher at large organizations - Stake v Rank.
The small organization means that there are no “group projects” so every PM is directly responsible for the program they’re working on and you can’t have slackers.
I suspect the small size also helps keep PM quality high. Quality tends to slip when there is flexibility in who you can hire (DARPA is incredibly flexible with who it hires to be program managers) and there is pressure to hire a lot of people.
The downside of the tiny flat organization is that there don’t seem to be any checks on the Director’s power, so if they don’t like a project or a person they’re out. This is why politicization seems dangerous. Starting in 2001, DARPA directors began to sync up with presidential administrations. It would mean that the director prioritizes aligning their goals with the administration outside of the organization more so than the institution’s nominal goal.