Question

Why earth isn't getting bigger even though the continents are spreading apart

Answer 1

Earth is not getting bigger because the continents are simply slowly moving in one direction. The mass stays the same all throughout the process. All of the continents are inside of our atmosphere, not connected to it, and the continents are not getting bigger, nothing is. They are simply moving slowly. 

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Answer 2

The earth isn't getting bigger even though the continents are spreading apart is because the continents are simply being shifted from one location to another, but no land or mass is being created. The mass of certain parts of the world is simply being displaced from one location on the earth to another or its shape is simply being changed by being pushed up or below the earth's surface.