If divergent plates cause new land to be added to the Earth why doesn't the Earth just grow and grow?

While divergent plates do create new land through processes like seafloor spreading, the Earth doesn't continually grow because this new land is balanced by subduction, where tectonic plates are pushed back into the mantle at convergent boundaries. The Earth's size remains relatively constant as the material created at divergent boundaries is offset by the material lost at subduction zones. Additionally, the recycling of crustal material ensures that land is constantly being transformed rather than simply accumulating.