Source drive has total files size increased by ~x4

[msladepas]

My first creation of a external boot copy under Mac OS 10.5.6 had the problem that the SOURCE disk ("Macintosh HD") ended up almost full. I started with Macintosh HD having ~72 GB of files. I ended up with nearly 320 GB on it after the standard recommended path of backup to external FireWire 800 drive: erase target disk, then copy files from Macintosh HD, then "Do Nothing".
Details: this was my work machine, not the laptop I am writing from. It's a Mac Pro 3.2 Intel desktop. I used SuperDuper! 2.5. The target was a LaCie 301832 320GB "Rugged All-Terrain" drive, and, as I said, I used the FireWire 800 interface.
In the end, after deleting a few large files for safety and rebooting, the MacPro was back to ~72 GB on the internal drive (not a RAID drive or anything exotic). However I feel I must have done something wrong since this does not happen when I use SuperDuper! at home (separate license) with my MacBook Pro. It was difficult to come up with a title that conveyed the problem.