Plate boundaries are the ways in which the Earth's crustal plates interact with each other. This is large-scale movement of the enormous solid plates resting on liquid rock. As these plates move, the topography of the Earth changes. A Convergent Boundary is where one plate pushes underneath another and becomes part of the molten rock underneath; the plate is reduced in size as this occurs. The Pacific Ocean Plate is the largest in the world, but it is slowly decreasing in size as it subducts under the Philippine Plate. As the Pacific Plate decreases, the Pacific Ocean shrinks because the sea-floor is reduced. Reciprocal movement on the Atlantic Ocean side of the Earth is causing the Atlantic Ocean to grow in size.
To simplify, Convergent Boundary movement is causing the Pacific Plate to subduct underneath the Philippine Plate, reducing the size of the Pacific Ocean.