If, according to Technorati, the blogosphere is israel email list still producing 1.5 million+ posts each week, that's conservatively growing the web by ~20 billion pages each year. It should come as no surprise that Google, along with every other search engine, has absolutely no desire to keep more than, possibly, 10-20% of this type of content (and anyone who's tried re-publishing in this fashion for SEO has likely felt that effect). Claiming to have the biggest index size may actually be a strike against relevancy in this world (according to Danny Sullivan, it's been a dead metric for a long time).
So - long story short - Google (very likely) has a limit it places on the number of URLs it will keep in its main index and potentially return in the search results for domains. The interesting part is that, in the past 3 months, the number of big websites (I'll use that to refer to sites with an excess of 1 million unique pages) we've talked to, helped through Q+A or consulted with that have lost wide swaths of indexation has skyrocketed, and we're not alone.