I can actually see a lot of room in the box for shrinkage. Hell I see a lot of room for cost reduction. However I would like to say everyone is being narrow minded literally. In other words everyone is thinking that a redesign must be a slim model. When in fact it is far more practical for a new model to be a short model. In other words the reduction isn't in the width, but in the length.
I see no major hurdles for Microsoft merely cutting two inches off of the right end of the console, and moving the components there closer to the drive. There is already two inches of bare space in place on the console. You may even get more by reducing the footprints of both the power button, and the controller ports. They can even remove one of the memory card slots. Lets face it they are not terribly relevant these days with internal memory, and low cost hard drives. The reality is that with cooler boards the machine doesn't need to be as spread out as it once was.
Hey I call dibs on calling it the X360 short.