That is, new and low traffic sites are crawled by less intelligent bots, and as the site gets more visitors or better rankings, more complicated and resource intensive bots are deployed.
How this might work with the most popular sites out there, the Amazons and Wikipedias of this world - I'm not so sure about that. If I were in charge, I'd be tempted to have customised bots and ranking weights for each of these exceptional sites.
Sadly the chances of getting a real answer on this in my lifetime are close to zero.