We reveal that concurrent events of inherent entropy boosting and increased synchronization between A- and B-site cation vibrations of an ABO3-type perovskite structure give rise to a larger piezoelectric response in a ferroelectric system at its morphotropic phase boundary (MPB). It is further evident that the superior piezoelectric properties of xBiNi0.5Zr0.5O3-(1-x)PbTiO3 in comparison to xBiNi0.5Ti0.5O3-(1-x)PbTiO3 are due to the absolute flattening of the local potentials for all ferroelectrically active cations with a higher spontaneous polarization at the MPB. These distinctive features are discovered from the analyses of neutron pair distribution functions and Raman scattering data at ambient conditions, which are particularly sensitive to mesoscopic-scale structural correlations. Altogether this uncovers more fundamental structure-property connections for ferroelectric systems exhibiting a MPB, and thereby has a critical impact in contriving efficient novel materials.