The assumption comes from simple math. If you have a 10 story building and make it twice as high by duplicating the bottom 10 stories, you've just doubled the

The assumption comes from simple math. If you have a 10 story building and make it twice as high by duplicating the bottom 10 stories, you've just doubled the number of living units. Considering that some buildings have large lobbies or retail in the bottom few stories you might actually be able to get o more livable units. It isn't like we are building pyramids. Yes, not 100% of the building isn't livable, and I don't think anyone implied that. But if you double the height, you should be able to double the living space. You don't need a ton more infrastructure to support the extra floors. Of course the double comment assumes you essentially copy the lower floors, and not make the upper floors into larger living spaces.