This is because of the presence of subduction zones, the destruction of old crust balances the formation of new seafloor, slowing the growth of the Pacific Ocean.
What are subduction zones?
Subduction zones occur all around the edge of the Pacific Ocean, offshore of Washington, Canada, Alaska, Russia, Japan and Indonesia. Called the “Ring of Fire,” these subduction zones are responsible for the world’s biggest earthquakes, the most terrible tsunamis and some of the worst volcanic eruptions.