If the tectonic plates are constantly moving why is the earth not getting bigger in diameter

From what I remember, as the earth's plates move constantly, this would mean some plates are moving apart which creates new lands under the sea. However, some other plates are sliding over one another, so one plate can sink down into the Earth's mantle. 

Hoped i helped CX

Iwould say d. eye contact and body language because you aren’t using any spoken communication. hope this : )

D.