The formation of particles in microemulsions is studied by Monte Carlo computer simulation. Two different types of particle size distributions can be obtained (unimodal and bimodal), depending on surfactant film flexibility and concentration. A bimodal particle size distribution is obtained only if two conditions are fulfilled: Growth by autocatalysis is the predominant process during nanoparticle formation and both growth processes (autocatalysis and ripening) take place at different times. The results allow us to explain the appearance of bimodality in recent experimental results.