If your System Protection backup is using VSS snapshots to support multiple restore points, then the amount of storage required will grow as data changes. For this reason, the capacity of the backup destination should be at least 2.5 times larger than the amount of data being backed up. If there is other data on the drive, then the free space needs to be at least 2.5 times larger than the data you’re backing up.
This allows room for the snapshot to maintain copies of data that has changed. As the extra space for these copies runs out, the oldest copies (restore points) will be deleted. This capacity will allow for multiple restore points..."
"...eine Festplatte, deren Speicherkapazität mindestens dem 2,5-Fachen der Größe der zu sichernden Elemente entspricht"