If you doubled the number of transistors in the same space then wouldn't that nearly double(quadruple) the heat output? How would someone even keep these things at a reasonable temperature at 7nm?
My limited understanding is that heat is related to the square of the voltage. And that by reducing the size of a transistor, you can lower the voltage. I don't think you can lower the voltage by quite as much as the reduction in size, so there must be a point when you can't just pack more in. Though gate design or choice of materials can reduce the voltage needed as well. They can also make the chip smarter about switching off parts that aren't needed at a given point in time to keep overall heat down.
