How is the diaphragm work to change air pressure in the lungs, and as a result, cause breathing?

Your lungs are located within your chest cavity inside the rib cage They are made of spongy, elastic tissue that stretches and constricts as you breathe. 

As your lungs expand, air is sucked in through your nose or mouth. The air travels down your windpipe and into your lungs. After passing through your bronchial tubes, the air finally reaches and enters the alveoli air sacs.When you breathe out, or exhale, your diaphragm relaxes and moves upward into the chest cavity. The intercostal muscles between the ribs also relax to reduce the space in the chest cavity.

As the space in the chest cavity gets smaller, air rich in carbon dioxide is forced out of your lungs and windpipe, and then out of your nose or mouth.

Why  do you think body composition focuses  on the  ratio  of  fatty tissue  to  other body tissue? why is  body composition not related  to  weight?