I find myself more concerned about what might be stored in your email to reach over 11 GB; it seems quite astonishing to me.
Here’s Google’s answer:
To delete a Google device backup, you could try opening the device’s Settings, navigating to Google > Manage backup (or something similar), toggling off the backup setting, and then selecting Turn off and delete when the prompt appears, which would aim to erase the backed‑up data from your Google account. You might also delete specific data such as photos and videos from Google Photos by turning off backup for that app and then using the Google Photos app or website to remove the files.
For an Android pphone backup:
Open Settings on your Android device.
Tap on Google, then All services.
Under “Backup and restore,” tap on Backup or Manage backup.
Find and toggle off the setting for “Back up by Google One.”
When prompted, select the option to Turn off and delete to confirm you want to delete the backed‑up data from your Google account.
