I’m actually surprised you are ranking that high already in google for such a competitive key phrase. Google is very tough to get ranked in for a new site due to their ageing filter. After you have been indexed by google for 6-8 months, you should naturally start moving up in the rankings once the filter is removed. However, you won’t get to the top results for that key phrase without doing a ton of links. At least that is my understanding.
Yahoo and MSN do consider backlinks as well, but their ranking algorithims are different than googles. You might want to check out seochat.com for more advice on all of this. Another goldmine of information.
As for the size of the site, I have been wondering this lately as well. We recently had 800+ pages indexed in google, but had found that many of our sub-pages weren’t linked properly for google’s spiders. We just completed an update, and I’m expecting to see about 2,600 pages indexed soon. The interesting thing is that google is currently updating their backlinks data and most of the new backlinks they are showing for us come from our own internal pages. Not sure what that means, or if we get any additional value from it, and I’ve heard that google’s backlink check isn’t very helpful since they only show a random slice of the information. Using Yahoo to check backlinks seems to be the standard.
The only think I know for sure (since it has been stated a million times on various forums) is that google loves content and the more of it you have, the more they like you.