So most of us know the basics of why and how we breathe. One thing we are not taught is that society has changed how we breathe. Corsets anyone? Why do you think we have furniture called "fainting couches" and such things as "smelling salts"? It was because the clothing of the time prevented breathing in such a way that ladies would literally pass out just to rest. Even in today's society it is considered unattractive to expand the belly when breathing. We have become hyper sensitive and self conscious about our tummies. In keeping our bellies flat (and usually sucked in), we change which muscles we use and the amount the lungs expand.
This picture shows using the diaphragm (a giant muscle that is like an umbrella under the ribs) to breathe. The diaphragm is the main muscle we should use for breathing.