Relaxing the diaphragm during expiration allows the lungs to recoil and regain the intrapleural pressure experienced previously at rest.
This phenomenon occurs because of the elastin in the elastic fibers in the connective tissue of the lungs, and because of the surface tension of the film of fluid that lines the alveoli.
Surfactant is a surface-active lipoprotein complex formed by type II alveolar cells.
By absorbing to the air-water interface of alveoli with the hydrophilic head groups in the water and the hydrophobic tails facing towards the air, the main lipid component of surfactant, dipalmitoylphosphatidylcholine, reduces surface tension.
It also means the rate of shrinking is more regular because of the stability of surface area caused by surfactant.