How does the Leap Motion device work? According to a recent article at The Verge they use "a number of camera sensors to map out a workspace of sorts".
The Kinect uses a micro-lithographed diffuser with an infrared light source to spray tiny IR dots all over your living room. The Kinect's camera then analyzes this image and reconstructs the 3D space. I find it hard to believe that Leap Motion is using this technology, partly because they must've made amazing improvements to the tech on their own, but also because they claim that using an better camera sensor (than the Leap's VGA sensor) is all that it would take to make the device's capture field grow from 3 cubic feet to "thirty feet, or three hundred."
My guess? They're using a light-field camera for their design. That, plus some great software, would work well as well as scale.