I had an issue of filling up my entire backup drive as well. Upon checking on my main computer (containing the AC database), it was the Backup folder that had grown to over 300 GB, not the Imported Items folder.
I used Amazing Utilities (under Advanced Options "Cleanup & delete outdated backups") to delete many of the Backup files. This reduced my AC backup to just under 60 GB.
You have to keep deleting old Backups in the AC folder. Even if you have Amazing Backup backing up to another hard drive, it seems to put a copy in the Backup file on your main computer.
