A thread at Search Engine Watch Forums named Google Index Size Numbers have decreased discusses a theory that Google has lost about 50% of its index. The logic behind this theory, also better explained here, is that when Jean V�ronis recorded some figures on stop words, and then compare the same word count to yesterday, there has been a drop of about 50%.
Seems like a logical theory but it is also important to take other considerations into count. We know Google has been recently aggressive with its duplicate content filtering. Maybe they are simply not showing the count of certain results that might match that criteria based on the duplicate conditions. The threshold is now much more tight? Maybe...
