A systematic study of the size dependence of the photoluminescence spectra of Si quantum nanocrystals in the SiO2 matrix has been performed. The results have been fitted to a quantum-confinement model that includes the nanocrystal size dispersion rather than a specific size of the nanocrystal. This, in conjunction with the results [Z. H. Lu, D. J. Lockwood, and J.-M. Baribeau, Nature 378, 258 (1995)] for amorphous Si layers serves as a strong confirmation of the confinement-induced nature of the photoluminescence.