Dense colloidal suspensions, whose volume fraction is larger than 5%, are encountered in various research fields, such as slurry in chemical engineering, soymilk in food science, and intravenous fat emulsion in medical pharmacy. It is significant to assess non-destructively the particle properties of the suspensions, such as particle size distribution in the sub-micrometer scale and dispersion degree. Near-infrared spectroscopy has the potential to enable the assessment based on a relation between light scattering and particle properties. The suspensions strongly scatter light. However, the assessment is still under development because the relation has not been fully understood yet. In a dense suspension, the interference of the electric fields scattered by the particles strongly influences the light-scattering properties, the so-called interference effect. Since the 1980s, many researchers have studied the interference effect in suspensions at different volume fractions using the dependent scattering theory (DST), which is the electromagnetic theory. However, the mechanism of the effect needs to be clarified because of their complicated dependence on the particle size and its distribution, optical wavelengths, etc. We developed the DST in a polydisperse system to examine a relation between the particle size distribution and light scattering properties. Our numerical results showed that the logarithmic distribution strongly influences the scattering properties. We also developed an analytic model equation for the volume-fraction dependence of the scattering properties. The equation allowed us to evaluate the interference effects on the particle properties rapidly using a single parameter. Our results are indispensable for developing near-infrared spectroscopy using scattered light.