Structural subnanovoids in silica-based glasses with various glass former contents are systematically studied using positronium (Ps) as a probe. The average sizes of the structural subnanovoids are estimated from the 2D-ACAR (two-dimensional angular correlation of positron annihilation radiation) momentum distribution of para-positronium (p-Ps) and from the pick-off annihilation rates of ortho-positronium (o-Ps) in the subnanovoids. The sizes of the subnanovoids decrease almost linearly with the glass former molar content. This finding gives us useful and unique information about the structural subnanovoids and suggests the possible tool for the study of the stability of nuclear radioactive waste glass during long-term storage.