Although still in its design concept phase, the Flash Bag inflates like a balloon to represent total stored capacity. So, as the USB based flash drive is filled with data a built-in pump (assumption) fills the surrounding bag with air inflating it. The idea is that you can, at a glance of course, look at the Flash Bag to determine if any space remains.
More product info here.
Flashbag – The Inflating USB Flash Drive [EverythingUSB]