The SharePoint Quota shown in the Microsoft Admin Center does not represent the amount of data that is actually available on the sites or can be backed up. The Quota includes a significant amount of content that is not part of the site’s actual file structure. The Quota value increases, for example, due to:
- all versions stored in version history
- the contents of the Recycle Bin and Second‑stage Recycle Bin
- the Preservation Hold Library (if enabled)
- SharePoint’s hidden system libraries and other background data maintained by the platform
These elements can increase the Quota value substantially, even when the site’s actual content is much smaller. Therefore, the Quota does not reflect the amount of data visible in the site’s root structure (Root folder) or the data that can be backed up via the Microsoft Graph and SharePoint APIs.
The combined size of the Root folders typically corresponds to the data that can be backed up, which is why the Root folder size and the backup size are usually – as in your case – very close to each other, around 9 TB.
In summary:
The Quota can be many times larger than the actual Root folder content because it includes a large amount of SharePoint background content that Microsoft does not expose through backup APIs. The backup size reflects the true amount of data in the Root folders, and a large Quota value does not indicate that anything is missing from the backup.