Emphysema patients have a higher metabolism. In order to meet the higher oxygen demand of the tissues, they must bring in larger volumes of air.
The tumors associated with this disease obstruct the airways. The expansion of the chest cavity helps draw air past these obstructions.
Emphysema patients have less respiratory surface area because of the damaged alveoli. To compensate, they must bring in larger volumes of air when they inhale.
The diaphragms of emphysema patients become paralyzed. The voluntary actions used to increase chest volume help compensate for this problem.