Windows 7 Backup Utility backes up every file, every time

I used to use Windows Vista backup, and it wasn't perfect but it got the job done for me.

So now I've set up Windows 7 Backup to run once a week to an external hdd on a home server I have set up on my local network. So anyway, that all went well and it backed up the roughly 55 gigs I was expecting it to back up.

The problem I have is that the next week, when I have it regularly scheduled to backup again, it backs up an ADDITIONAL 55 gigs onto the hdd. After it did that, the next day I manually told it to backup again and it didn't' even finish because there wasn't' enough room on my 150gig hdd!

Anyone else use this utility? Any idea how to get it to only back up the NEW or CHANGED files (so I only end up with a 500mb or so backup every week instead of 50gig)

And no, I'm not having it back up a system image, just my libraries and a couple of select folders on my hdd that aren't covered in the "libraries"

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