Look at the deposition pattern of inhaled particles.
The largest particles which reach the lower airways deposit within the intrathoracic trachea (impaction). A 10 µm particle delivers 1000 times as much of a drug as a 1 µm particle, which has relatively little deposition within the airways. Hence the largest intrathoracic airways, which have a smaller total surface area than small airways, receive most of the drug. It is therefore to be expected that the mucosal concentration of a drug administered by aerosol will be highest in the largest intrathoracic airways.