When new oceanic lithosphere is generated at an oceanic ridge, what happens to the plate it's attached to?

The plate gets larger.

The process of forming new oceanic lithosphere which happens in mid-oceanic ridges is called seafloor spreading. A new crust is created by means of volcanic activity which is the result of the dissolving of the oceanic lithosphere or descending plate.

When new oceanic lithosphere is generated at an oceanic ridge, the plate gets larger it's attached to.

A mid-ocean ridge or mid-oceanic ridge is an underwater formation, fashioned by tectonics. This rise of the ocean floor happens once convection currents rise within the mantle to a lower place the oceanic crust and make rock wherever 2 tectonic plates meet at a divergent boundary.

Answersource rock;