It takes a lot of self-discipline to leave a bag with significant empty space.
Thanks for the compliment, David. But really, I just pack what I really need, and it doesn't fill my bag. Putting more things that I don't need in the bag just because there is room just adds weight I don't want to carry. I've had a lot of problem finding a bag small enough to be completely filled that still has the features I want (opens like a book, hide-away straps, cinch straps, etc). Seems everyone thinks an 1500 ci bag should open like a day pack.
But to the original question, I think the previous post a just speculation from people who don't have a rolling carry-on. The bottom wall of the bag has to be pretty stiff to keep it from sagging over the wheels, and it looks like it has several rigid molded parts. If you put it on its back, does it collapse to flat, or does it retain its shape? It might not "squeeze down". On the other hand, 1" is not a lot to squeeze it.
Let's hear from someone who actually has the bag and knows because has tried to compress the bottom wall.
BTW, the Rolling Backpack is only 7" thick (and lighter, too).