They use a different kind of tank that prevents any gasses from escaping in the PZEV regions ('clean states' ie dirty states) and I suppose the design is such that it holds less fuel in the same space because of the walls needing to be able to hold a certain amount of pressure.

Same kind of difference between the North American Prius II and the Japan Prius II. 2006 HCH will be PZEV in all North American areas, so it's going to have a tank size in between the current small and large tanks.

 

I do feel guilty I'm spamming godknowswhat nasty gases in exorbitant amounts into the atmosphere but I gotta say - it does feel nice using those extra gallons to the very max to lengthen those tank trips.