You can see how the batteries fit snuggly behind the stepper motors. By placing the batteries and the motors in the lower part of the mouse, it keeps the center of gravity low.

Larger wheels would have helped our mouse, you can see how low to the ground the chassis lies. Although it did fine in our test maze, Dexter got hung up twice on seams in the mazes of other schools. If you're going to take your mouse to other mazes, expect the worst.