Retina MacBook Air vs. rMBP 13

Hi guys, I'll make it quick. Lots of people seem to be expecting a retina MacBook Air. How would that add up? How would they be able to make it thinner than the 13" retina MacBook Pro yet fit a large battery in there? Without an optical drive in either model, I'm wondering how they will be able to differentiate the two product lines. Ideas?