6. Conclusion
This paper has studied the properties of mechanisms for selling content ads on the Google Display Network where multiple advertising opportunities on a page are auctioned off at the same time and displaying less than the maximal number of ads means the ads that are displayed can be dynamically resized and shown more prominently, thereby inducing these ads to receive more clicks. These position auctions with dynamic resizing have not been previously considered in the academic literature. In a standard position auction with no dynamic resizing, the optimal mechanism is a VCG mechanism with a reserve price equal to the optimal reserve price for single-slot auctions. With dynamic resizing the publisher will instead want to select a configuration that is efficient with respect to the virtual valuations rather than the actual valuations. The optimal mechanism under position auctions with dynamic resizing will typically result in fewer ads being displayed than would be displayed under the VCG mechanism with reserve prices. Furthermore, if the publisher uses the VCG mechanism with reserve prices, the publisher will want to set a higher reserve price under dynamic resizing than when there is no dynamic resizing, and the optimal reserve price will vary with the number of bidders and the amount of dynamic resizing that takes place.
