I could not find out how many pages Google indexed over time but I found the self-reported numbers of Google’s timeline. Here they are in a convenient table:
|When||Number of web pages indexed|
|May-June 2000||1 billion|
|November-December 2000||1.3 billion|
|July – August 2002||2.5 billion|
|November – December 2002||4 billion|
|January – February 2004||4.28 billion|
|November – December 2004||8 billion|
|August 2005||8.2 billion|
It seems, roughly speaking, that the number of pages indexed doubles every 16 months or so though the growth seems to be sometimes sudden. It is likely that the number of pages indexed depends on hardware and Google probably purchase its hardware in large bundles. Also, competition from other search engines is likely to be a factor.
Update: I’m sorry. I was motivated partly in my quest by a post by Will which I read earlier today. I should have given him credit. Will also points out that the index size as reported by Google needs to be verified. I really like this link Will sent me about why the search problem is hard.