After their original post, there was apparently some controversy about the methodology Matthew Chenay & Mike Perry used to compare the relative index sizes of Google & Yahoo! (original article at http://vburton.ncsa.uiuc.edu/indexsize.html, update at http://vburton.ncsa.uiuc.edu/indexsizepage.html.)
After some modification of their search algorithm, they came to a similar conclusion: Google DOES have a bigger index than Yahoo! Previously, the difference was almost 3-to-1 (37.4%), but now it’s more like 3-to-2 (65%). Still, that’s pretty significant when Yahoo! is claiming their index is twice as big as Google’s!
Another interesting fact is neither search engine had an actual total number of results that was anywhere near the estimated total number of results. Even including duplicate results, Google’s actual return was only about 54.5% of the estimated total, while Yahoo!’s actual return was only about 22.6% of the estimated total. If you take out the duplicate results, the discrepancy between actual and estimated is even worse!