The theory of diffusion of both nonionic and ionic surfactants in micellar systems is extended to the case with variable aggregation numbers. General relationships for the mobility and diffusion coefficient of surfactants are derived, and their dependence on the aggregation number variability is analyzed. Theoretical reasoning is supplemented by numerical calculations on the example of the sodium dodecyl sulfate micellar solution. It is shown that the effect of the aggregation number variability can be significant even for spherical micelles when investigating the surfactant diffusion in the vicinity of the critical micelle concentration.