That's not how it's supposed to work, the nids will attack every single territory (or there is a chance of the nids attacking each territory, something like that), so if you control half of the map you will have half of the nids bitting your ass, in theory you could hold very little territory and be the wining faction (because you are just focused on your campaign objectives and denying enemy objectives instead of holding as much territory as possible) with barely any nids after you. The devs have talked about this in a couple of threads but I can't find a source right now.
Basically,this pool http://forum.eternalcrusade.com/threads/what-race-will-you-play-at-launch.170/ is why we are assuming SM will overpopulate.
"It sucks that you're grounded and can't go out with your Warrior Brood. Maybe I could be your Spawning Pool tonight? Norn queen will be gone all night. She'll never know"
While I definitely appreciate your sentiment regarding "dev attention", I think it just really boils down to how attached people are to various lore. Also, you have consider that, in Warhammer 40k, the Space Marines are realistically the "main" faction with which a lot of the stories are told. That or Imperial Guard, but who wants to be a countless mook? So, even though each faction will likely get plenty of development attention, you still can't account for the fan bases desire to play the faction they are most familiar with. On a slight side note, I almost feel like playing Eldar would be like playing Vanu in PS2. Interesting aesthetic, but underpowered due to lower population counts =(. But who knows, maybe the character and weapon designs will be super sick and draw more players.
