Hyperinflation or hyperaeration is the condition of the lungs, particularly the alveoli or air sacs overfilling with air during inhalation, resulting in abnormally large lung volume. On an x-ray this will make the lungs look less substantial (brighter) as the x-rays will pass through the lungs more easily, there will be fewer landmarks on the x-ray indicating the parts of the lung, and the diaphragm will be abnormally depressed.
Although hyperinflation can be a sign in increase in lung mass, it's generally the result of a partial obstruction or closure of one of the large airways, congenital lobar emphysema, or a mucus plug from asthma.