Since you seem to be all knowing about Apple's "DNA", remember the keynote where Steve Jobs unveiled the iPad, sat down in a comfy chair, crossed his legs and demonstrated typical usage of the iPad on stage?
The original iPad weights 1.5 lbs, the iPad air cut that down to 1 lbs. A 12" iPad could probably done at around 2 lbs, which wouldn't be that much more than the iPad 1 and still lighter than a MacBook. With typical usage (which doesn't always involve holding it with one hand), it wouldn't make that much of a difference, while providing more screen estate.
This whole idea that "If it weight the same as a MacBook air, get the Macbook because you can do more stuff with it!" seems to completely disregard the touch screen interface paradigm as something that is inferior in almost every way to the Mac's windowed interface, as an obliged compromise that exist only to enable more mobility.
I didn't buy an iPad because it was lighter than a MacBook. I bought it because I prefer the direct manipulation aspect that multi-touch screen devices bring, along with the simplified "no-hassle" philosophy of iOS. So of course, like many others, would like to have more of it by the means of a bigger screen size.