If the earth’s crust is growing at mid-ocean ridges, why doesn’t the earth grow larger?

Because as that grows the size of the center of the earth where it is supper hot isn't change so the part of the earth closet to the center gets small and  smaller with the amount of mistrial allowing for new growth to slowly do the cycle for example you plant a tree it grows big and dies and soon goes back into the earth and as more things die on top of it  slowly pushes the die tree to the center