True, but size will shrink in time and will also become more efficient. This device is new but it still greatly increases the amount of time you can stay underwater. Using an air tank you will have to get a new tank every 40 minutes or so. Tanks are huge. Here you just need a new battery or a power cord.
Plus with 1 minute recharge batteries:
http://www.toshiba.co.jp/about/press/2005_03/pr2901.htm
SOLID STATE BATTERIES
And even fuel cells
The technology could evolve to let you stay down there all day. When SCUBA diving, 40 minutes goes by very fast.