In many studies on emulsion polymerization emphasis is on a constant average particle size and an average monomer concentration for all particles, and the zero-one or pseudobulk approximation is applied. Based on kinetic Monte Carlo simulations, it is demonstrated that the kinetics need to be followed particle by particle while acknowledging the difference in radical and monomer concentration per particle, in contrast to the conventional Smith-Ewart methodology. Focus is on miniemulsion free radical polymerization, with physically reasonable values for all parameters and considering interphase mass transfer of initiator and monomer molecules and initiator-derived, monomeric and oligomeric radicals, following a diffusion-based approach. The temporal evolutions of the chain length distribution (CLD) and the particle size distribution (PSD) are correlated and affected by radical exit/entry and monomer mass transfer. The results unveil unprecedented insight into dispersed phase radical polymerizations and open the pathway to a joint PSD and CLD control and design.