If new crust is being created as magma cools into solid rock, why isn't the size of the earth increasing (getting larger) ?

Erosion/weathering

Explanation:

Erosion occurs when weather/other natural agents (wind/rain/hail etc.) removes or transports the ground's soil/rocks. The magma that cools into solid rock will eventually erode. This is the main reason as to why our planet is not increasing in size.