1. A computer-implemented method, comprising: receiving expandable content item requests for publishers, each request specifying a publisher resource address of a publisher resource and a content item environment in the resource, the publisher resource address defined by a resource path that includes a domain name of the publisher; for each content item request, providing request data for a prediction specifying an expansion direction for the content item environment based on position data for the resource and the content item environment, the request data specifying the publisher resource address of the resource and the content item environment in the resource; for each prediction for which a successful prediction of a predicted expansion direction occurs, selecting an expandable content item having an expansion direction that is consistent with the predicted expansion direction; for each prediction for which a successful prediction does not occur: determining, from position data from a plurality of other resources each having a resource path that includes the domain name of the publisher, a prediction specifying an expansion direction for the content item environment; and selecting an expandable content item having an expansion direction that is consistent with the predicted expansion direction; and for each content item request, providing response data in response to the content item request that causes the selected expandable content item to be provided for the content item environment.