Voltage fade significantly hinders the practical use of Li-rich Mn-based layered oxides (LLOs) as cathode materials for next-generation high-energy-density Li-ion batteries. Therefore, an in-depth understanding of the factors influencing the LLO voltage fade during cycling is fundamentally important for tailoring the structure and thus improving the electrochemical performance of the corresponding electrodes. Herein, we compare the electrochemical performances of LLOs with different particle size and conduct in situ high-pressure response measurements to determine the effects of particle size on voltage fade, demonstrating that small particles can undergo a reversible layer-to-spinel phase transition that results in improved voltage stability during cycling. The above finding provides a novel paradigm for the development of high-capacity LLO electrodes and thus contributes to the establishment of a more energy-efficient and green society.