It is true that new crust is being created at the mid-oceanic ridges, but this does not make the Earth any bigger despite the the growth of oceanic crusts in these regions.
The reason that Earth does not get bigger is because the same plates that are being created at the divergent plate boundaries are recycled into the mantle at the subduction zones. Because of this, the surface area of the Earth remains the same.
lets say we have a big lake with a lot of animals and you want to move the water to somewhere else. A lot of animals and fish, frogs will have to find a new home!
