I was just about to ask the same question myself. I have the 690 with 12 TB protecting 6 TB of data protected. This is taking up 8.5 TB local storage on the appliance and 3.3 TB in the cloud and I have been running the appliance for a year. My suspicion is we are not getting the de-duplicatation or compression on the appliance as we think or at all, but it is being de-duplicated in the cloud.
There have been times where I have taken several snapshots of my servers over the course of a couple of weeks and I notice a remarkable similarity in the amount of disk space I am taking up on my storage compared to the Barracuda appliance. If I am to believe my dashboard figures that say I am getting an 82% reduction in my data then my local storage should be way less that it is now.
My Day 1 backup with no DD should be 6 TB and with an 82% reduction that the Dashboard says I am getting it should reduced to 1 - 1.5 TB. With an appliance retention policy of 7 days local/off, 4 weekly local/off, and 12 monthly all off, 7 yearly all off, I should have about 12 incremental snapshots on the appliance and about 24 in the cloud by now. Lets say the 1.5 TB DDup is correct and I have a change rate of about 10% per snapshot (very high) then 3.3 in the cloud is not unreasonable.
But I can't see how we are getting DD on the appliance when I have 6 TB of data and only incremental changes for up to 4 weeks and it is taking up 8.5 TB on the appliance. That is about 140% of the protected storage. I would suspect I would take up the same amount of storage on a USB device if I did a full uncompressed backup and added 12 incremental over the same period. Even back when I was using Backup Exec to tape I would easily get 1.6 to 1 or better with just standard compression and I am not getting anywhere near that on the appliance.