If you've ran the backup utility more than once, chances are it's versioning your backups.
My backup runs every night at 9pm. I am beginning to think its making multiple copies similar to the multiple restore points windows can create. Copies of copies of copies.
I currently have all my important files in Dropbox and older copies are actually on my HDD. I'm tempted to delete the external backup file and start over. The only things on my HDD are a few gb of photos, the programs and windows. All of the programs can be downloaded again if need be and the photos are also on multiple devices and a DVD or two.