We have recently succeeded in loading extremely small monodisperse gold clusters (1.2 ± 0.3 nm) as cocatalysts on a water-splitting BaLa4Ti4O15 photocatalyst using a glutathione-protected Au25 cluster (Au25(SG)18) as a precursor; an improved photocatalytic activity of 2.6-fold was obtained when compared with that of BaLa4Ti4O15 onto which large gold nanoparticles (10–30 nm) were loaded. In the current study, the controlled loading of a series of ultrasmall Aun clusters onto BaLa4Ti4O15 using various Aun(SG)m clusters (n = 10, 15, 18, 22, 25, 29, 33, 39) was examined. The results revealed that the use of a highly stable cluster as a precursor is essential for achieving control over the loading of the gold clusters. Additionally, the origin of the improved photocatalytic activity owing to the ultra miniaturization of the cocatalyst was reconsidered herein based on the photocatalytic activities of the obtained photocatalysts. The results strongly suggested that the activity per gold atom on the surface decreased significantly owing to the ultra miniaturization of the cocatalyst and that the origin of the improved activity is the increase in the number of surface gold atoms at a rate that overcomes the reduction effect in their activity.