Hi Rap,
MoodleCloud offers a list of all files that are counting towards your quota which you can access by clicking on the "USERS" and "QUOTA" graph at the top right of any page (next the messaging icon), and then clicking "See all".
From here the list can (in some cases) take you to the rough location in Moodle where those files are used; which should help you delete the files (by clicking the "View in context" button). Unfortunately due to the way Moodle stores files it is not always possible to show precisely where they are used.
Cheers,
Cam