If you have a busy and full inbox in Outlook, you can see how much space each folder taking up. Here’s how to find them so you can clean out the full folders.
Click on File to access Backstage View. If you have multiple accounts setup, select the account you want to check.
Then go to Info >> Cleanup Tools >> Mailbox Cleanup.
In the Mailbox Cleanup window click View Mailbox Size.
In the Folder Size window you’ll see how much space each folder is taking up.
Now that you know which folders are taking up the most space, you can clean them up.