Thank you for an interesting and insightful question.
I think ww gave an excellent answer, but I'd like to add a few observations.

First, it's not really "fair" to compare the sizes of the various orders, because they are so very different in many ways. What they do is different, and they have wildly different histories, including particularly their rates of induction and dismissal of members. They also have very different levels of appeal to the general population.

I think it's safe to say that pmdev has been very popular, in part because membership in pmdev allows people to peak behind the curtain of PerlMonks' workings. I dare say many of the people who have joined pmdev did so solely to view code, not to contribute changes. The various orders have different levels of power, so the gods induct new members into them with different degrees of solicitation and vetting. Janitors have a lot of power, and so are held to very high standards of conduct; abuse of this power can (and has) resulted in summary, permanent dismissal. Similarly for Power Users. But only a handful of Power Users are needed, because of what they do; and having too many can actually lead to conflicts. In contrast, a pretty good battalion of Janitors only serves to make their job easier.

Other orders — SiteDocClan, pollsters, Pedagogues, etc. — aren't quite so restrictive; primary qualifications are a burning desire and a modicum of trustworthiness and PM experience.... but they just don't have as much broad appeal. That said, not all applicants are accepted. This can be due mainly to a lack of workload. The workload of SiteDocClan, Pedagogues, and QandAEditors currently is quite low — easily enough for one or two dedicated persons to handle. This has been different in the past, and, @Larry willing, will be different again in the future. :-)

See Cabal Matrix to get a real idea of how active the members of the various orders are.

(Note that this discussion does not cover the fraternal orders, which have no official role or power in PerlMonks.)