The indentation-size effect (ISE) in a commercial SiAlON was investigated by conducting Knoop indentation between 0.1 and 20 kg. The resulting ISE was analyzed utilizing Meyer's Law, Proportional Specimen Resistance (PSR) model, and a Multifractal Scaling Law (MFSL). Meyer's law and the PSR model fits to the hardness–load data were not excellent. Further analysis based on the PSR model and MFSL revealed three piecewise linear fits corresponding to load regimes 0.1–0.3, 0.5–2, and 5–20 kg. Physical inference of MFSL fit parameters suggested that these three load regimes correspond to where indentation behavior is governed by deformation mechanisms limited to single grains, grain boundaries, and multiple grains, respectively. Independent of the ISE analysis results, comprehensive examination of indents by scanning electron microscopy revealed that changes in deformation mechanisms could also be grouped into these three load regimes. Corresponding changes in deformation mechanisms were microcleavage cracking, grain-boundary cracking, and macrocracking, respectively. These observations are consistent with the findings of both the PSR model and MFSL with respect to the physical aspects of the governing mechanisms. It is concluded that these mechanisms are responsible for the observed ISE in this commercial SiAlON.