Semiconducting quantum dots (QDs) have been considered as promising building blocks of solar energy harvesting systems because of size-dependent electronic structure, e.g. QD−metal heterostructures for solar-driven H2 production. In order to design improved systems, it is crucial to understand size dependent QD−metal interfacial electron transfer dynamics, picosecond processes in particular. Here, we report that transfer rates of photogenerated electrons in Pt-decorated CdS QDs can be varied over more than two orders of magnitude by controlling the QD size. In small QDs (2.8 nm diameter), conduction band electrons transfer to Pt sites in an average time scale of ~30 ps, giving a transfer rate of 2.9 × 1010 s-1 while in significantly lager particles (4.8 nm diameter) the transfer rates decrease to 2.2 × 108 s-1. We attribute this to the tuning of the electron transfer driving force via quantum confinement-controlled energetic off-set between the involved electronic states of the QD and the co-catalyst, respectively. The same size-dependent trend is observed in presence of an electron acceptor in solution. With methyl viologen presented, electrons leave QDs within less than 1ps from 2.8 nm QDs while for 4.6 nm QDs this process takes nearly 40 ps. The transfer rates are directly correlated with H2 generation efficiencies: faster electron transfer leads to higher H2 generation efficiencies. 2.8 nm QDs display a H2 generation quantum efficiency of 17.3% much higher than that of 11.4% for 4.6 nm diameter counterpart. We explain these difference by the fact that slower electron transfers cannot compete as efficient with recombination and other losses as the faster transfers.