There are at least tens of millions of web sites. This site ranks 3500 on Alexa and 12k on Compete. Google knows that and prioritizes its crawlers accordingly.

There are at least tens of millions of web sites. This site ranks 3500 on Alexa and 12k on Compete. Google knows that and prioritizes its crawlers accordingly. Maintaining a fresh index of the top 100k sites is where search engines get their best bang for the buck.