The disk usage amounts displayed on the "Service Usage" screen in Users & System Administration are totals of the data being used in the attachment, audit log, and database storage areas.
The "Service Usage" screen shows the overall disk usage for your entire domain and the disk usage for each service.

To reduce your disk usage, delete unnecessary attachments, which are saved in the data area for attachments. Deleting attachments reduces your disk usage, and the freed up space is used for storing new data.

The data area for attachments stores files attached to "Attachment" fields in apps and files attached to thread comments. Deleting attachments reduces your disk usage, and as a result, your available disk space increases.

The database area stores the following data:

• Input data except for attachments
• Setting information

The database area expands automatically in accordance with the size of the data stored in the area. Once the area expands, it does not shrink even after the size of the stored data decreases. Therefore, deleting data in the database area does not reduce your disk usage. The space freed up by deleting data in the database area is reserved for reuse.

To reduce the disk usage, delete unnecessary attachments. Deleting attachments reduces your disk usage, and the freed up space is used for storing new data.

Note that if the Change History feature is enabled for an app, deleting attachments in records of the app is not enough for reducing your disk usage. Since the attachments remain in the history, consider deleting the files in the history.
Deleting Files in the History of an App

The files in the history of an app are also counted toward the disk usage.
If the Change History feature of an app is enabled, attachments remain in the history even after you delete them from records in the app. To delete attachments that remain in the history, you must perform one of the following actions:

• Deleting records
• Deleting the "Attachment" field
• Disabling the Change History Feature

When the Change History feature is disabled, the history recorded up until that point is deleted. The history deletion may take some time and thus the disk usage may not be reduced immediately.
Disabling the Change History Feature

Adding, updating, or deleting a large amount of data frequently and repeatedly with APIs or other means results in the increased number of audit log entries. If the disk usage is still high even after you delete attachments or disable the Change History feature, try shortening the retention period of audit logs. This may reduce your disk usage.
Configuring Retention Periods and Export File Formats of Audit Logs