False Full only 16GB instead of c.50GB

[mac_dave]

*****
Using superduper v. 2.5
Mac OS 10.4.11
Maxtor One Touch 4 HD using firewire 400
*****

well the other night i set up superduper to backup my mac mini (80GB HD).
when i checked the superduper in the morning it had backed up only 16GB, which it said was a full bootable backup.

superduper was running from a non admin account. the mini was set to stay awake and it was in the morning. i used cocktail to run scripts , maintenance and repair permissions on pilot. restarted the mini just in case. logged in holding down the shift key.

my itunes library is bigger than 16GB alone. its located in public folder, for sharing between accounts.

So what went wrong, how do i fix this?