The solidification of the deep and global magma ocean 4.5 billion years ago likely stands as the most significant differentiation event in the Earth’s history, setting the initial conditions for its subsequent evolution1. However, our understanding of this solidification process remains limited, partly due to a lack of knowledge regarding the grain size of Earth’s most abundant solid phase, bridgmanite2-5. In this study, we employ a combination of cutting-edge techniques, including machine learning potentials, seeding, and enhanced sampling, to investigate the crystallization of bridgmanite using large-scale molecular dynamics simulations consisting of up to 1 million atoms. Our primary focus is on elucidating the key parameters controlling nucleation and grain growth processes, in particular the least-constrained crystal-liquid interfacial energy. Our results demonstrate a significant pressure-dependent increase in interfacial energies of bridgmanite-liquid, surpassing those of silicate-liquid systems at ambient pressure by a factor of up to ten6-8. Within a convective magma ocean, this amplified interfacial energy allows bridgmanite crystals to reach at least decimeter-scale regardless of the nucleation mechanism, driving efficient fractional crystallization and causing substantial chemical differentiation and mantle compaction. The large crystal sizes imply the existence of highly viscous convective cores of the lowermost mantle, promoting the preservation of primordial signatures in Earth&\#039;s deep interior. Our study sheds light on the enigmatic solidification of Earth’s magma ocean, offering crucial insights into the early evolution and compositional heterogeneity of our planet.