Why is my backup file larger than the capacity of all backed up hard drives?

Solution 1:

Bottom line it appears Window has this notion of backup periods that seems consistent with what Frank Thomas commented above. Seemingly a file can be backed up multiple times across a set of backup periods.

So it looks like when the disk filled up there was this option to Check backup disk space with its Options button.

The next window that displays is the Manage Windows Backup disk space: Select how disk space is used by Windows Backup which gives you a summary of how much space occupies the data file backup, any system images, and other files you happen to have on the disk.

Then when you click on the View backups button under the section

Data file backup: You can free up disk space by deleting data file backups

You will find the following view with a table presenting each backup period and its size:

Select a backup period to delete

Deleting previous backups will free up space on your backup location. All files backed up during the selected period will be deleted.

I'll look into running some backup experiments to see how changes through time get captured across these backup periods.