Does size matter when it comes to the number of indexed pages in a search engine? Who knows, but even though Yahoo! has often said it does not, it did not stop them from bragging about their bigger size. According to their blog, they say, As it turns out we have grown our index and just reached a significant milestone at Yahoo! Search - our index now provides access to over 20 billion items. While we typically don't disclose size (since we've always said that size is only one dimension of the quality of a search engine), for those who are curious this update includes just over 19.2 billion web documents, 1.6 billion images, and over 50 million audio and video files. Compared to Google's stated size of over 8 billion, that is a sizable difference!
They also say that as with all index updates we are still tuning things so you'll continue to see some fluctuation in ranking over the next few weeks, so be prepared for some rankings changes.