The signal of small particles is superimposed by the signal of larger particles
With dynamic light scattering the particle size is obtained from the variation of the scattered light signal over time (see Dynamic Light Scattering).
Besides the speed (respectively frequency) of this intensity fluctuation, also the time-averaged scattering intensity depends on the particle size. Since the intensity of the scattered light increases with the 6th power of the diameter, superimposes the scattered light of a few large particles the signal of the small particles. This results in a limitation of dynamic light scattering: Comparatively few large particles superimpose the scattered light of nano particles so significantly, so these cannot be detected.