This study shows how the simple modulation of the cathode/anode mass ratio, in a Li-ion capacitor based on activated carbon (AC) and Li4Ti5O12 (LTO), results in a drastic increase in performance. Starting with a device balanced in the classical way (with an AC/LTO mass ratio of 4.17), the cathode/anode mass ratio has been reduced to 1.54 and then to 0.72. At a high power density, the device with a cathode/anode mass ratio of 0.72 shows the highest energy density. In fact, at 2.3 kW L−1, it delivers an energy density of 31 Wh L−1, which is almost 10 times greater than the energy obtained with a capacitor balanced with an AC/LTO ratio of 4.17 (3.68 Wh L−1). Moreover, the reduction in the cathode/anode mass ratio from 4.17 to 0.72 improves the cycling stability with a factor of 4.8 after 1000 cycles at 10C. Electrochemical impedance spectroscopy reveals that the better power performance is due to reduced diffusion and charge transfer resistances. In addition, the anode polarization is less pronounced for the system with a lower AC/LTO mass ratio, leading to a minimization in electrolyte decomposition on the anode surface and therefore limiting the increase in the electrode resistance during cycles.