RPS isn't that big either. They only have such huge numbers because they have
gameplay events which are advertised on the RPS website. Their actual numbers (of active players) are much lower. I rarely see them running more then one platoon (from what I can tell).
Looking at their stats, SYND also has a lot of inactive members.
So no outfit on Miller is actually that big. Sure, those are the larger ones but they are far less in numbers then their member count makes them look like.