Deep-focus earthquakes occur at 300-660 km depth. Geophysical observations and deformation experiments propose the olivine-spinel (wadsleyite/ringwoodite) phase transformation as the faulting mechanism. While geophysical observations indicate that fault geometry influences the b values in the Gutenberg-Richter law for the phase transformation faulting, deformation experiments reveal that b values are also influenced by rock properties, including structural heterogeneity. Grain sizes play a crucial role in the rate of phase transformation, impacting the occurrence of faulting. Consequently, grain sizes may also influence b values. We conducted deformation experiments on germanate olivine, an analog silicate olivine material, with various grain sizes to reveal the effect of grain size on the difference in b value during the phase transformation faulting. We used a Griggs-type deformation apparatus and measured acoustic emissions (AE) with an AE transducer, which was calibrated by laser-doppler interferometry. This calibration enabled the acquisition of AE waveforms with a unit of velocity (m/s), facilitating comparison to natural earthquakes. b values in the fine-grained aggregates (a few μm) are smaller than those in the coarse-grained aggregates (hundreds μm) at the same deformation conditions. In the coarse-grained aggregates, the heterogeneous formation of spinel aggregates contributes to high b values. Conversely, in the fine-grained aggregates, the homogeneous formation of spinel grains inside olivine at the grain boundaries results in lower b values. Therefore, the homogeneity (or heterogeneity) of spinel formation appears to be a controlling factor for b values in phase transformation faulting associated with deep-focus earthquakes.