Here's why:

* Seafloor Spreading: New oceanic crust is indeed formed at mid-ocean ridges. This process, known as seafloor spreading, pushes existing crust away from the ridge.

* Subduction: However, this new crust eventually gets recycled back into the Earth's mantle at subduction zones. These are places where tectonic plates collide, and one plate slides beneath the other.

* Balance: The creation of new crust at mid-ocean ridges is balanced by the destruction of crust at subduction zones. This dynamic process keeps the Earth's surface area relatively constant.

Therefore, the Earth's radius and surface area do not increase significantly due to seafloor spreading. The process is essentially a cycle of creation and destruction.