With a smaller population of users, each interaction becomes an investment in an individual relationship that may pay off later (financially or otherwise). And if someone upvotes your comment, you know they are endorsing you, personally.
With a larger population, this dilutes to something like a branding exercise.
So there's some balance between having lots of diverse perspectives and skill sets (large population) vs. having individualized relationships (small population).
Similar to the idea that companies may have an optimal size, maybe there's an optimal size for social news sites. And for social news sites where utility is part of the draw (HN), this optimal size may be smaller than sites oriented more around entertainment (digg, reddit).