If new ocean crust is constantly being made, why aren't the oceans getting bigger?

According to Plate Tectonics theory, there are seven large plats on the Earth’s crust that are moving on a large scale, creating the phenomenon called continental drift. The reason why oceans do not become bigger is that the old crust is destroyed as it goes under in the subduction zones and to the mantle. According to estimates, the Atlantic Ocean is spreading, but the Pacific Ocean is not since it is undergoing subduction in many of its boundaries. 

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