Hello Amburguesa, and welcome to the Apple Support Community.
The existing replies correctly distinguish iCloud Drive from iCloud Photos, but one additional point may explain the 300 MB of “Documents” usage.
The Documents category in iCloud Storage isn’t limited to files visible in iCloud Drive. It can also include application data stored in iCloud by third-party apps or Apple apps using CloudKit. Some of this data isn’t directly browsable through iCloud.com or the Files app, even though it legitimately occupies iCloud storage.
To identify what is using that storage, check Settings > [Apple Account] > iCloud > Manage Storage on the iPhone. If an app is listed there, disabling its iCloud storage (if appropriate) may remove the associated data after confirmation.
If no apps account for the reported storage and iCloud Drive remains empty across the iPhone, Files app, and iCloud.com, it may be worth contacting Apple Support, as they can verify whether the storage report is accurate or if there is an issue with iCloud storage accounting.
Helpful links:
Manage your iCloud storage on your Apple device - Apple Support
iCloud User Guide - Apple Support
iPhone User Guide - Apple Support
If you discover which app or service is using the reported 300 MB, please post an update.
Your findings may help other members of the Apple Support Community who encounter similar unexplained iCloud storage usage.
Please let us know the outcome.
Your feedback may also help other members of the Apple Support Community working with similar issues.
🙏🏻
Have a good Day.
And one more thing…
Never forget to think different.