Poromechanical effect, adsorbed-layer thinning, and desorption-induced shrinkage processes in porous organic matter (OM) change pore sizes during depressurization of shale reservoirs. While the first two processes have been studied in the past, desorption-induced shrinkage has not been studied in detail. We develop a model that includes all of these processes to study the dynamics of pore size change with pore pressure. We further relate change in OM pore sizes to the change in dynamic porosity and apparent permeability (AP) of OM. The ability of a poromechanical process to contract pores diminishes as pore pressure decreases. On the contrary, the ability of adsorbed-layer thinning and desorption-induced shrinkage to enlarge pores increases as pore pressure is reduced. Pore size change in smaller pores is more sensitive to poromechanical and adsorbed-layer thinning processes. Dynamic porosity and AP can change as much as 10% and 15%, respectively, by reducing pore pressure from 50 MPa (e.g., initial reservoir pressure) to 4 MPa (e.g., economic limit). The contribution of each process depends on the initial pore size distribution (PSD) of the system and pore pressure. At pore pressures greater than 20 MPa, the poromechanical process dominates and decreases porosity and AP; in other words, porosity and AP decrease as initial reservoir pressure reduces to 20 MPa. Below 20 MPa, desorption-induced shrinkage and adsorbed-layer thinning processes shift the balance to increase porosity and AP.