A 58-year-old man with a 40 pack-year smoking history presents with progressive dyspnea. Pulmonary function tests show decreased elastic recoil of the lungs. On examination, he has a barrel chest and hyperinflation on chest X-ray. The point marked **D** on the lung compliance curve diagram represents the equilibrium position where inward elastic recoil of the lungs balances outward elastic recoil of the chest wall. Which of the following best explains the rightward shift of point **D** in this patient's respiratory mechanics?