I have a 768MB VPS, on which I host 1 website. That website has WordPress installed and a MyBB forum and it fills 22 GB in space. It shouldn't have more than 1 GB. And a few weeks ago it had 15 GB in space, so it's increasing even though I haven't added/uploaded anything. The email service is disabled and I've looked through the files on the website and didn't saw any file that could be responsible for so much space usage. Where else can I look?
Search for large logs. Also check to see if anyone has uploaded large (or multiple) files through the forum somehow.
Try backing up your server to your computer and see which folders are holding that much data. Also some servers backup the files to a archive which is on the server, which might increase space usage.