I don’t really know what’s gone wrong here – 11GB is rather excessive! I’d suggest resetting the cache (just close the solution and delete the redacted folder, or click the “Clear caches” button in the ReSharper → Options → General options page), and reloading the solution. The cache should be considerably smaller. It would probably be worth updating to the latest version of ReSharper (9.1.1 is the latest released, 9.2 is in EAP builds). If the cache is still this big, contact support, either by emailing support at jetbrains dot com, or by raising an issue directly at https://youtrack.jetbrains.com
EDIT (23/02/2018): Removed the section on integration with Windows disk cleanup. Sadly, this isn’t a completed or released feature.