Large Crawl Store DB

hi -  i've got 72,000+ of these errors in my search logs:

The object was not found. ( Error from SharePoint site: HttpStatusCode NotFound The request failed with HTTP status 404: Not Found.

on the database, i noticed the  Search_Service_Application_1_CrawlStoreDB_GUID database swelled to 157GB, completely filling up the database drive.   any ideas why this is and how i can resolve?  thanks