Air flow is proportional to the pressure drop along the airways and inversely proportional to resistance.
Resistance is impacted by viscosity, airway length and diameter of airways.
The most important parameter is airway diameter. That is because resistance increases as the radius increases to the 4th power. That is, if the radius is 1 the resistance is 1. Thus any change in airway diameter will impact flow dramatically. This intrapleural pressure keeps the lungs expanded.