We investigate the physical mechanism governing the sigmoid-like time-averaged response of stochastic magnetic tunnel junctions (s-MTJ), which is a promising building block for probabilistic computers. We measure the time-averaged resistance of perpendicular easy-axis s-MTJs with various free-layer thicknesses and diameters as functions of an external magnetic field and current. The time-averaged response shows no significant dependence on the free-layer thickness, whereas significantly varies with the diameter. Based on the Néel-Arrhenius law, we derive an analytical expression of the time-averaged response against both the magnetic field and current and discuss the underlying mechanism accounting for the obtained results. We show that the experimental results are well explained by considering magnetically active and electrically active volumes of the superparamagnetic free layer in s-MTJs. The obtained finding provides an important design guideline of s-MTJs for probabilistic computers.