This is because Google is double dealing, they are selling both side of the transaction, on the search side they have side bar ads and in search result ads, but

This is because Google is double dealing, they are selling both side of the transaction, on the search side they have side bar ads and in search result ads, but on the destination, they also sell the ad impressions. So they are using their monopoly power to make money on both search and the display on the destination. So they have a huge incentive to push sites with lots of pages, more opps to get ad clicks on those big site than the small ones.