Matt Cutt answers some of the questions that have arisen lately:
Firstly, Google is not full. The problems of web sites dropping of the index and coming back during Google’s BigDaddy upgrade were not due to lack of machines; the crawl/index team certainly has enough machines to do its job, and we definitely aren’t dropping documents because we’re “out of space.”, according to Matt.
The problem of pages dropping from the index were due to bad inbound link quality
The sites that fit “no pages in Bigdaddy” criteria were sites where our algorithms had very low trust in the inlinks or the outlinks of that site. Examples that might cause that include excessive reciprocal links, linking to spammy neighborhoods on the web, or link buying/selling. The Bigdaddy update is independent of our supplemental results, so when Bigdaddy didn’t select pages from a site, that would expose more supplemental results for a site.
Enclick’s philosophy about linkbuilding is: there is no such thing as a free lunch.. An easy link-exchange with somebody you don’t know is more likely to be a bad quality link, which might even get you de-indexed.
Your inclusion ratio also depends on the strength of your inbound linking. Google has a threshold of inbound link strength below which Googlebot just shallow crawls your site.
From Matt Cutts: Gadgets, Google, and SEO � Indexing timeline