Will Prius Gen II battery work in Gen III?

Last year I bought a nice, new Optima battery for our Gen II Prius....now we've just went and bought a new 2012 Gen III car. I'd like to switch out the battery ...transplant the Optima from the Gen II to the Gen III. I think the Gen III current battery is stock from factory and at almost 5 years old, it will surely go out faster than the Optima. Will it fit no problem...or are they different sizes? Thanks in advance!