The equilibrium among differently sized micelles of nonionic linear surfactant chains, as well as their shape and density profiles, have been studied using Monte Carlo (MC) simulations and self-consistent field (SCF) calculations. The SCF theory predicts a far lower critical micellization concentration (cmc), a larger aggregation number at the cmc, and a sharper interface between the lyophobic and lyophilic segments in the micelle, as compared to the MC results. The mean-field approximation used in the SCF theory, and in particular its effect on the chemical potential of the bulk state, is the main factor contributing to the discrepancies in the cmc. The difference in aggregation number and sharpness of the lyophobic/lyophilic interface are caused by the neglect of micellar shape fluctuations in the SCF theory. Such fluctuations are shown to be more important for surfactants with equal head and tail sizes than for surfactants with a larger head than tail group. The equilibrium micelle size distribution is used to study how the standard chemical potential of surfactants in aggregates of different sizes depends on the aggregation number.