Oil, here is another analogy for you to attempt to get your head around. Take a litre of water and boil it in a saucepan, eventually that litre will not exist in the saucepan any more as it will evaporate. Pumice is formed by the molten rock "boiling", gases are escaping and only when the molten rock cools does it form bubbles. Now you might state the rock is expanded, however the amount of Gases that have evaporated into the atmosphere are high. Obviously the Earth's core and Magma are continuously hot enough to evaporate gases, some energy is lost into geothermal gieser's. This loss means that if there was even the smallest growth in regards to what you suggest, it would be lost elsewhere. I guess you can say the Earth is "Flat" as opposed to "Open".