There has been lots of speculation in the past about Google Increasing Index Size Again. As a present to us, Google says, in appreciation of Google's Official 7th Birthday, Google says, "for our seventh birthday, we are giving you a newly expanded web search index that is 1,000 times the size of our original index." In that, they link to a new page posted at http://www.google.com/help/indexsize.html notes, "search engines' published metrics for index size measurement vary greatly and are no longer easily comparable."
A must read on the topic are the following articles; - Google Announces New Index Size, Shifts Focus from Counting by John Battelle. - End Of Size Wars? Google Says Most Comprehensive But Drops Home Page Count by Danny Sullivan
The thread discussing this at the moment is at Search Engine Watch Forums.