The issue of population is, likewise, an important one in the background of any setting - to me, it's a good marker of how much thought the creator or creators put into it, or how much of a sense of scale they have. There's a natural tendency for writers to think that "bigger is better," and to do so with populations in order to establish a more "epic" or "sprawling" feel. Sometimes this works. Other times, it's like taking a sledgehammer to the plate glass window that is willing suspension of disbelief. It's a tempting trap, one that I've had to carefully avoid a few times while I've been putting things together.
I try to take my craft seriously, and that's why when I encounter something that goes against that, it's like steel wheels squealing on the rails.
I recently picked up StarCraft II: Wings of Liberty, Blizzard Entertainment's latest real-time strategy game and the culmination of twelve years of anticipation and expectation. What I've already noticed is that it greatly expands the setting of the Koprulu Sector, a galactic fringe tens of thosuands of light-years away, colonized by exiles from Earth - and I think that one of its big issues, story-wise, is that it's kicked the "bigger is better" issue into overdrive. I'm not sure whether or not it's unfamiliarity on the part of the writing team with what was previously established, a desire to change what has already been established about the setting, or just not caring about inconsistencies, but in my opinion it does diminish StarCraft II as a work.
To summarize: the original settlement of the Koprulu Sector was made by thirty-two thousand terrans, then spread across three planets, and totally isolated from one another for at least sixty years. Based on what information I've been able to find online, they've had two hundred and forty-five years to increase their numbers. If you assume that their population doubled every twenty years, which is pretty good considering these are exiles on totally alien planets with an extremely limited technological infrastructure, by the time of the setting's "present day" the population should stand at a bit over 131 million. Which is not bad - I can certainly see that sort of population managing, with the aid of a great deal of sophisticated technology and equipment, to keep a multiplanetary civilization holding together. Plus, a not-too-big population adds to the "frontier" atmosphere that many space-based settings try to elicit, and if there's never enough people to do everything that needs doing, individual heroes have the chance to make a difference.
With StarCraft II, it's clear Blizzard didn't go this route. Korhal, the capital world of the Terran Dominion, looks like Coruscant and is listed as having a population of 6.3 billion. What makes this even more hilarious is that the setting background included the nuclear destruction of that planet, thirteen years before StarCraft II's present day, in which its entire population was wiped out - its population of four million.
Where, in short, are all these bloody people coming from? Artifical wombs? Clone farms? What?
All of the people in this photograph were created, educated, and released into the world in 4.2 seconds thanks to the power of SCIENCE!
Working within limitations is one of the greatest advantages any creator has. I know well how much it sucks to be spoiled for choice, such as when I've got multiple potential plots vying for my attention and can't narrow one down enough to start work on it, and so none of them make any progress. Ignoring them, or pretending that they don't exist, may not be one of the Great Sins of worldbuilding but it's still damn annoying. When I pick up your book, turn on your movie or boot up your game, I want to be immersed in a world that I can believe in. Details like this, details that don't even need to be in the first place - which population numbers definitely are - only shatters that belief for me.