Symptoms
Cloud storage is overused by backups of M365 (Teams, One Drive, etc) or Google Workspace (Google Drive).
Backup storage size is several times bigger than the backed up data.
Cause
There are multiple causes why the C2C backups consume significantly more storage space than the data being backed up. Data handling works differently because of some peculiarities of the source storage itself.
1) Data deleted from the source is still included in the backup