Electrocatalytic water splitting for hydrogen production is a crucial technological approach for renewable energy storage and utilization. Based on the hydrogen evolution reaction mechanism, this paper systematically reviews the structural evolution of hydrogen evolution reaction catalysts from bulk materials, multidimensional nanostructures, to nanoclusters, single atoms, and single atom-nanocluster synergistic systems from the perspective of size regulation of active components. Extensive literature studies demonstrate that as the size of active species decreases, the metal atom utilization efficiency of catalysts significantly improves. Nanocluster and single atom catalysts exhibit intrinsic activity surpassing traditional bulk materials due to their unique quantum size effects and coordination environments while reducing noble metal usage. Notably, the synergistic system of single atoms and nanoclusters effectively promotes water molecule dissociation and hydrogen desorption through a dual-site mechanism, significantly enhancing alkaline hydrogen evolution reaction kinetics. Based on this, future research should focus on multi-scale structural regulation and precise synthesis, deeply elucidate the dynamic structure-activity relationships during the reaction process, and thereby design low-cost catalysts with high activity, high stability, and broad pH adaptability.