The top ad-supported sites’ revenues are growing quickly and they are beginning to run out of space. Meanwhile, smaller sites can’t seem to unload their inventory.

This imbalance has existed since the earliest days of the web, but it is now clear that the recovery will not be evenly distributed.

There is an opportunity to arbitrage this gap in the price of exposures to the same people from different sites. Some of this will be resolved as advertisers get smarter about the way they buy online advertising. Also, a lot of the slack will be taken up by keyword-oriented advertising systems from Overture, Google, and a few late entrants.

Unfortunately,this still won’t solve the revenue gap for most mid-sized web sites.