That's a great way to put it, T'Girl. Thanks for the perspective! I hereby rescind my 'limitless energy' claim.
I took it as 150+ individual species. Humans are a member. Vulcans are a member. Trill are a member. Betazeds are a member. etc. The vulcan colony of (insert name here) isnt`a member of the Federation, the Vulcans are a member of the federation. Unless a colony separated to form its own government, and then subsequently that government decided to join the federation, then it wouldn`t be considered a member in its own right.
I don't think that's necessarily true. First of all, the klingon and Romulan empires are considerably older than the Federation, and obviously expansionistic. They probably have lots of very old, well established colonies, meaning the total number of Klingons or Romulans is probably very high. Many of the Federation member worlds, on the other hand, very often seem to be very anti-expansionistic. I would not be even remotely surprised to learn that the majority of Fed. Member species have no colonies outside their home system. Even the vulcans, andorians, etc, seem to have very few colonies considering how long they've been in space. Usually it seems like Humans are the only driving force behind the expansion of the Federation. On top of that, there's also evidence that the Klingon and Romulan empires are very much not single species entities. The Romulans clearly use alien slave labor (and every Romulan not working in the mines is another romulan available to man the Fleet, the Tal Shiar or the research labs), and if they're willing and able to do so on Remus, the same is almost certainly true at other locations as well. The Klingons in TOS are shown as being just as interested (and with roughly equal chances of success) in getting random alien races to pick their 'side' as the Federation is, which suggests that the Klingon empire may actually have any number of loose vassal states, and that's on top of all the other worlds the klingons have conquered over the centuries (conquest being repeatedly referred to as a major historical pattern of the empire). There's also the 'North Korea' factor to consider: even if the Federation is huge compared to the other empires, that's not a major advantage at all if the vast majority of its people is just sitting around enjoying civilian life with no interest in Starfleet, while the Romulan and klingon societies are built from the ground up to maximize their military potential. Especially in consideration of the fact that those empires don't even need to be capable of completely conquering the Federation in order to be a terrifying threat to it, because a peace-loving society like the Federation is terrified at the very possibility of war. All they have to do is be threatening enough to make it absolutely clear that any war between them would bleed the Federation badly. And none of that even touches on the Technological playing field - cloaking devices are an amazing advantage in and of themselves.
Correct. Ezri Dax was from a non-Federation planet, but it's doubtful that she had to require sponsorship (as Nog had to) in applying to Starfleet.
I would describe Humans as obviously expansionistic, and as the main driving force behind federation expansion. I'm not so convinced about the rest of the Federation, especially if there are only 150 member worlds despite us having seen so many species. And the point still remains that both Empires have existed much longer, and therefore had much more opportunity to expand.
Masao Okazaki's The Starfleet Museum postulated a Federation history where the Human race reproduces so fast and founds so many colonies, that it begin to cause problems with other Federation members. Especially given that Humans in that history basically dominates Starfleet and we use it to assist our colonial expansion.
I do think a high human population would explain why they have so many more colonies than the other races. Maybe most races in the UFP are in the low billions or less than a billion, while humans are a few dozen billion? Still less than the Husnock were.