But it does not sit on top of the earth. It remains above the earth as a heavenly body and replaces the sun.
Speaking of huge, we seem to forget that the New Jerusalem is also 1500 miles high. Which means that if it were a layered city (which is entirely possible) it would have many more that 1500 layers. By the same token we are speaking of heavenly things, so we not not really know the details. But the cubic capacity of this city could easily exceed the linear capacity of all the continents on earth.
