As Samsung have shown with their Q35, you can have more or less exactly what's in the Macbook, improve on the battery life even, and still shave a whole pound off while making it a little more compact (and keep the cost the same).
It is within Apple's design capabilities, but perhaps given the debacles with the Macbook and Macbook Pro, they are gauging how much miniaturisation is within their reach while retaining acceptable build quality.
I'd love one, but not if it's built like the MB/MBP (MBP especially).