In recent years, several groups have invested significant research capacity to investigate the effect of the metal crystallite size on the various primary and secondary reactions taking place during the Fischer–Tropsch synthesis. Most publications agree that with decreasing crystallite size (dcryst<10 nm) the surface-specific CO hydrogenation activity (turnover frequency) decreases. In parallel, an increased selectivity to the undesired methane was observed. In the present study we combined and extended previously published data on alumina-supported cobalt nanocrystallites with observations on an alumina-supported cobalt carbonyl complex ((CO)9Co3CCOOH) and its decomposition products under Fischer–Tropsch reaction conditions. We could show that with an increased degree of sintering of the complex, that is, a larger cobalt crystallite size, the turnover frequency increased and the tendency to form methane decreased. The results connect seamlessly with those obtained for the nanocrystallites suggesting that the catalyst performance features extend into the sub-2 nm range.