If you could send the SuperDuper!.log to support (it's off your Home folder in Library/Logs), I'll take a look at it and see what was going on at the time you canceled it.
Since we -- meaning SuperDuper! -- are only dealing with the DMG itself, and don't back up directly to the network device, it's hard for me to explain why so much space was "missing" from the NAS. It's likely that the time was being taken during the conversion and compression of the image, which takes 2x the size of the original or so (see p21 of the User's Guide for an explanation ).
Rather than using "direct to DMG" backups, I'd suggest following the steps in this FAQ:
http://www.shirt-pocket.com/forums/showthread.php?t=81
That'll provide you with an image you can subsequently update. I've done extensive testing here on a NAS (a Buffalo Linkstation) and it works just fine...