Question

why does chest cavity becomes smaller during exhalation

Answer 1

When you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. This increases the space in your chest cavity, into which your lungs expand. The intercostal muscles between your ribs also help enlarge the chest cavity. They contract to pull your rib cage both upward and outward when you inhale.

Answer 2

this decreases the space in chest cavity and forces out the air of the lungs