I've had shrinkage problems with great Stuff as well, and I was using the misting method to ensure quicker cure time, I've noticed when I use moisture to cure Great Stuff, I get this problem a lot more, or when I use it and there's high humidity in the air I get shrinkage, my thoughts are that when the humidity goes down or the air temp drops, the moisture in the bubbles condenses out of the air in the bubbles creating a vacuum and causing my project to shrink. When the temp goes back up the project expands again. So I try to only use Great Stuff in extremely dry environments. In the Summer this means I create a small workspace in my basement with plastic and place a dehumidifier in there set to 30% or less humidity. These projects never give me problems with shrinkage.
Walking in darkness, with only the flame of my desire to light the way