I guess, but would you consider the existing 13" MBP to be "significantly" thicker and heavier? There's already very little differentiation between the models.
I bought a 13" MBP to replace my 11" MBA because I was tired of waiting for retina and the size/weight difference seemed negligible. Regretted it within a couple of days of just using it around the house.
On a purely numbers basis, the difference doesn't seem significant (hence how I talked myself into the purchase), but there's a massive "human scale" threshold that's crossed in that difference. IMO, an 11" MBA feels more "like an iPad" in those respects vs. a 13" MBP very much feeling "like a computer".
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.
Yes I would. I know the difference looks small on paper, but I find the difference huge in day to day use. For example I can curl up on the sofa and comfortable use my 11" MBA cradle in one arm, I can't do that with the 13" MBP.