Are you editing any of your video files? How big is your email file?
Onecare backs up whole file again if the time stamp changed, or at least a single line is added.
Just start and incremental backup and see how much it is taking. You can check which files are in by running a scan from Backup Configuration page. (Change Settings --> Backup Tab --> Configure Backup --> Change File Type --> See which file type and how much amount is going in this incremental backup)
This gives some understanding of what is going in as part of daily backup.
If you want a quick fix, make sure yuo don't have any missed file (you can check in restore wizard) and format the hard disk and make a new backup which will be again 40GB, I hope next incremental backup will be more meaningful.
