It is a lot if it's a 16:10 screen rather than the 16:9 on the current MBA 11
if Apple is minimizing costs ( for lowest prices laptops) it more likely going to be 16:9.
Also for a fixed diagonal 16:10 screen will be smaller than a 16:9 on. A taller screen would push the opposing corners farther apart so would have to shrink to keep the same diagonal.
The other 12% and 7% larger is for diagonals floating inside the "12" range; not the same size.
If Apple had contractors shave 9-10% off the 16:10, 13.3" retina screens they are using now that would be a 11.97-12.1" diagonal. That rounds to 12" nicely. If that reduction pushes that derivative into the same price zone as the old screen they'd go that way. If not more pixels would get trimmed off to fit the cost profile.
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Well, the 12" (really 12.6") might be a replacement for both mba's in the nearby future.
12.6" is big enough to perhaps drop both 11.6 and 13.3 and just have one MBA in the middle (coupled to bringing the rMBP 13" into the classic 13" laptop price point). Simplified product offering lets them keep the costs down. if Apple can squeeze out the screen costs there isn't alot of upside with sticking with just a 11.6" screen size at all. Apple going to the same screen tech cut to different sizes is a likely cost control path for them.
Having three 13.3" models are doomed. But even in Apple's drive to simplicity even two 13.3" isn't stable for the long term.
P.S. I think 12.6" comes from trying to minimize the bezel on the classic MBA design. IMHO that is bit backwards. The bezel is large because of the minimal constraints on keyboard (i.e, human hands). Over time the screen tech will get affordable enough to fill that up. It may not be the right time to do that now though. MBA unit numbers are going up because it is much more affordable now than it once was. Affordability is a constraint also.