Which process causes hillsides and mountains to gradually get smaller?

The answer is erosion

erosion and land water cause the ground to do either two things.

erosion causes the soil on top to be picked up by moving water or pounding and splashing water from the rain. this soil is usually moved to the bottom or to the lowest level around it.

land water, which causes the soil under to become softer and is able to become more and more compact and able to turn to petrified soil or even rock.