Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O-2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O-2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O-2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.