The sizes of more than 100,000 molecules in organic crystals have been assessed as the volumes of molecular Voronoi-Dirichlet polyhedra. The average molecular volumes for all crystals are shown to be nearly equal to the corresponding values in homomolecular (consisting of identical molecules) crystals. The validity of the Voronoi-Dirichlet approach in determining molecular sizes is substantiated and the reasons for the variations in the molecular volumes are discussed. It is shown that a molecule increases its volume if it is surrounded by a good deal of high-row (i.e. an element with more than ten protons) atoms or if there is disorder in the crystal structure.