Porous silicon formed by anodization of a p-type silicon substrate is characterized by a distribution of crystallites with diameters smaller than about 100 Angstrom. The corresponding size distributions obtained from Raman measurements show distinct peaks which are explained for the first time by the tunneling of holes through crystallite barriers during the formation process of porous silicon. The theoretical description is based on quantum mechanical calculations of the tunneling probability of the holes through small crystallites into the electrolyte. This tunneling probability shows oscillations as a function of crystallite size which are comparable to the structures observed in the size distributions, The calculations presented provide a deeper understanding of these size distributions and of the basic formation mechanism of porous silicon.