I don't buy that at all. A reasonable sized cubicle has a wide desk and space to move you chair in and out of, plus a 'corridor' between all the cubicles. Change cubicle walls to office walls and have doors open into the existing corridor, and it's no different in size. Building codes might require some changes but I don't think it will be as much as 2x.

The literature (Spolsky lists it one of his articles) implies that increased productivity of the existing employees in their own offices more than compensates for the slight decrease in the number of people you can pack into a fixed area.