It would be a better comparison in some ways, you're right. Still, when I've picked up a 13" MBA, it's felt more similar to an 11" MBA than a 13" MBP to me, in large part I think because of the difference in thickness: both MBAs are "0.3-1.7 cm" while the 13" MBP is 1.8 cm all the way through. The weight also puts the 13" Air halfway between the 11" MBA and 13" MBP.

Not knowing more about the specs of this hypothetical 12" machine, I'm theorizing it'd be approximately halfway between the 11" and 13" Air in size/weight. That of course may not be right, but it seems unlikely to be way off. So, while my comparison is hypothetical and imperfect, I don't feel it's quite apples and oranges, either.

Which is exactly why an 11 inch retina would sell well. There is no small pro.