Sitecore CMS stores the BLOB field values for media items in the Blobs table of the database. Old rows are not removed from the Blobs table when detaching media, reattaching media and deleting media items, which may lead to orphan information being stored in the table after some time.
To remove orphan rows from the Blobs table, use the "Clean Up Databases" application from the Desktop client interface (Control Panel -> Database - > Clean Up Databases).