We demonstrate that the sequence of reactants plays a key factor in the final size of Au nanocrystals. The growth mechanism markedly differs with seed size and/or internal crystallinity. Conversely to what could be expected when the multiple-twinned particle (MTP) seeds are rather large (3.5 nm) they control the nanocrystal growth. When the single domain seeds are very small (1.5 nm) the majority of nanocrystals produced are large icosahedral (85%) nanoparticles as predicted from a theoretical model.