The comparative analysis of two micropore-size distribution functions (MSD), i.e., the gamma-type function and the fractal (proposed by Pfeifer and Avnir) one are presented. Theoretical studies performed for different models of the geometrical heterogeneity of microporous solids (characterized by the various intervals of the values of the slit-like half-width and the pore fractal dimension) show that the structural parameters of these MSDs are correlated. In other words, the gamma-type function can be simplified to the Pfeifer and Avnir one under some rigorously mathematical assumptions.