The 13" MBP is half a pound heavier than the 13" MBA because that is the amount of extra battery that is required to drive the retina screen. If they made a 13" retina MBA , it too would weigh a half a pound more.
As to thickness, the MBP is 0.71" thick and the MBA is 0.68" thick. (Not too mention the neverending chorus of forum members who swear they don't ever want thinner devices.)
One spec that rarely seems to be compared between the two machines is footprint. The 13" MBA is 114 square inches. But can anyone guess the size of the footprint of the 13" MBP...
There is never, ever going to be a retina MBA.