The most likely diagnosis is COPD. The spirometric findings, in conjunction with this patient's clinical history, are most consistent with this diagnosis. His FEV₁/FVC ratio is low, consistent with an obstructive lung defect. The FEV₁/FVC ratio is used to assess for airway obstruction; a value less than 70% (the lower limit of normal) is consistent with airflow obstruction. With evidence of obstruction, the degree of reduction in FEV₁ is then used to characterize the degree of obstruction. An FEV₁ of 50% to 80% of predicted is classified as moderately reduced, 34% to 50% of predicted is severely reduced, and less than 34% of predicted is very severely reduced. This patient's reduction in FEV₁ qualifies as moderately severe. Increased lung volumes with higher than predicted total lung capacity suggests hyperinflation, and high residual volume suggests air trapping from increased lung compliance. The lower than predicted DLCO suggests an effect on the lung parenchyma, which in this patient is most consistent with emphysema. This patient's clinical history demonstrates a progressive course of dyspnea, in contrast to an episodic course with resolution, which would be more compatible with asthma.

Bronchiectasis is a condition that shares features with COPD, including mild to moderate airflow obstruction on pulmonary function testing and an abnormal lung examination with wheezing and crackles. However, patients with bronchiectasis usually have significant coughing with daily sputum (often thick sputum) and airways that are easily inflamed and collapsible. Chest radiographs and CT imaging often show distorted airway architecture. This patient, in contrast, has features of indolent dyspnea, a significant smoking history, and relatively unremarkable chest radiograph, which is more compatible with COPD.

Idiopathic pulmonary fibrosis is a form of diffuse parenchymal lung disease. This condition more typically causes a restrictive pattern on spirometry with parallel decreases in both the FEV₁ and FVC, unlike in this patient who has a decreased FEV₁ but preserved FVC. Additionally, pulmonary fibrosis is more likely to result in decreased lung volumes in contrast to the increased lung volumes seen in this patient.

Obesity hypoventilation syndrome is characterized by fatigue and daytime somnolence in patients who are obese (BMI >30), and the diagnosis is confirmed by arterial blood gas testing showing daytime hypercapnia with an arterial PCO₂ greater than 45 mm Hg (6.0 kPa). However, pulmonary function testing typically shows a restrictive pattern without obstruction, with a decreased FEV₁ and FVC but preserved FEV₁/FVC ratio. Although this patient is obese, his clinical symptoms and pulmonary function studies are not consistent with a diagnosis of obesity hypoventilation syndrome.