We have studied aggregation of a lattice model for amphiphiles due to Larson using Monte Carlo simulations and single-chain mean-field theory. Near-spherical micellar aggregates form spontaneously in the simulations. In contrast to standard equilibrium theories, our simulations show a decrease in free monomer concentration above the critical micelle concentration. This phenomenon is probably due to nonideal mixing effects in solution. The structure of micelles of a given aggregation number predicted by the theory is in excellent agreement with simulation results. The single-chain mean-field theory also accurately predicts the critical micelle concentration. The mean-field prediction for the distribution of micelle size is sharper and is peaked at lower values than simulation results. We suggest ways to modify the theory to improve agreement with simulation.