"Bulk-sensitive" Ce 3d - 4f resonance photoemission (RPES) with an unprecedentedly high resolution has revealed a clear difference of bulk Ce 4f spectral weights of heavy fermion system CeRu2Si2 and CeRu2Ge2 in the region of the tail of the Kondo peak and its spin-orbit partner. The significant spectral difference in both materials is in strong contrast to the mutually similar "surface-sensitive" 4d - 4f RPES. The obtained bulk-sensitive spectra are well reproduced by a single impurity model. Consideration of the crystalline electric field splitting is important for estimating a realistic Kondo temperature from the bulk 4f spectra.