I use Acronis 7 and I'm sure it is a bit different,and I don't do incremental images. However the 11 gb may be that it has to rewrite the whole program that has been changed from your previous backup. Although that would not necessarily mean you were saving an extra 11gb. I am not certain that this is the correct answer but I would give it a try and then see how much bigger your new backup is to the old one.
I think incremental images are a waste of time and if you backup to the Secure Zone as you run out of room the oldest image is pushed out. I also backup to my second HD, but I delete the previous one first.I think it is the safest way and I have never had a problem.