Monte Carlo simulations were carried out to explain experimental results concerning the different sizes obtained for Ag and Au nanoparticles synthesized in microemulsions. Computer simulations allowed to study the interplay between the chemical reaction rate and the material interdroplet exchange, and their consequences on the mechanism and size distribution of nanoparticles synthesized in microemulsions. It has been shown that, although the material interdroplet exchange depends primarily on the flexibility of the surfactant film, a slow reaction rate leads to a more effective material interdroplet exchange for a given microemulsion. Two factors contribute to this result. Firstly, a slow reaction implies that autocatalytic growth takes place for a longer period of time, because there are available reactants. If the reaction is faster, the reactants are almost exhausted at early stages of the process. As a consequence, autocatalytic growth is only possible at the beginning. Secondly, a slow reaction rate implies the continuous production of seed nuclei, which can be exchanged between micelles due to their small size, allowing the coagulation of two nanoparticles (growth by ripening). Once again, this exchange can only take place at early stages of the synthesis when the reaction is faster. Both factors, autocatalysis and ripening, favour the slow growth of the biggest nanoparticles leading to the production of larger particles when the reaction is slower.