As an element ubiquitous in the Solar system, the isotopic composition of iron exhibits rich variations in different planetary reservoirs. Such variations reflect the diverse range of differentiation and evolution processes experienced by their parent bodies. A key in deciphering iron isotope variations among planetary samples is to understand how iron isotopes fractionate during core formation. Here we report new Nuclear Resonant Inelastic X‐ray Scattering experiments on silicate glasses of bulk silicate Earth compositions to measure their force constants at high pressures of up to 30 GPa. The force constant results are subsequently used to constrain iron isotope fractionation during core formation on terrestrial planets. Using a model that integrates temperature, pressure, core composition, and redox state of the silicate mantle, we show that core formation might lead to an isotopically light mantle for small planetary bodies but a heavy one for Earth‐sized terrestrial planets. Plain Language Summary: Planetary formation and differentiation are fundamental processes that physically and chemically shaped terrestrial planets. As a major element ubiquitous in terrestrial planets, the isotopic ratios of Fe are found to vary among different planetary reservoirs, reflecting the formation and differentiation processes that occurred on their parent bodies. Deciphering the information behind these variations require experimental and theoretical constraints on how the isotopic ratios of Fe evolve during different planetary processes. Here, we present a systematic model on how Fe isotopes behave during planetary core formation based on high pressure experiments. We show that Fe isotope fractionation between the mantle and core of terrestrial bodies depends strongly on the planet size. Key Points: We constrained the Fe isotope fractionation factor for the silicate mantle up to 30 GPa using a bulk silicate Earth compositionWe present a holistic Fe isotope fractionation model for core formation considering pressure, temperature, composition, and redox stateOur model shows that core formation can cause the mantle of small planetary bodies to be light in Fe isotopes, but heavy for that of Earth [ABSTRACT FROM AUTHOR]