When you configure outlook, you may be unknowingly subscribing to some RSS Feeds. You may be getting these feeds daily and eat up your outlook available space. If you think you don't need any of those RSS feeds, you can unsubscribe from them.
Please follow the below link to learn how to unsubscribe from any RSS subscriptions.
This link also explains two ways to do this:
1. Unsubscribe and delete all the contents in the RSS Fees Folder.
2. Unsubscribe and keep downloaded content there.