The cases were obtained by reader at MacRumors, and show the iPhone 5 case, the iPad mini case, the iPhone 4S case and the iPad case all compared to each other. The cases were apparently created by case manufacturers based on leaked models out of Foxconn factories.
The photo does a great job showing the relative sizes of the various devices. The largest case in the back shows the size of the current 3rd Generation iPad. That iPad houses a 9.7" diagonal display. The next smaller size case is for the rumored "iPad mini". The smaller iPad is believed to have a 7.85" screen with a smaller surrounding bezel while retaining the same aspect ratio and resolution (1024x768) of the iPad 2. Rou notes that the iPad mini headphone jack is positioned at the top of the case.
Apple plans to announce the next iPhone September 12, when all these rumors will be put to rest.