Hi Mark,
I am using the backup2workstation module too, and experienced a similar problem. It turned out that the data had been trying to backup to the workstation but the workstation was not switched on, so the backup continues anyway, but actually ends up backing up on to the server, this producing a rather large file when the backup succesfully gets to the desktop. Sorry I'm not too great at explaining things!
If you want to check this, open a SSH to the server and check in the folder that you usually backup to (Eg: /mnt/backup/), and if there are some files in there, even though the workstation is not mounted, then that's you problem, and you can delete the files.
My system was doing this over weekends, when the workstation was switched off, so the data backed up onto the server, but I managed to stop that with a little help, I told Cron to only run the Backup job on days 1-5. It also recently done it last monday as this was a bank holiday, so the workstation was switched off.
Sorry if I've gone on a bit!
HTH
Matt