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Why can high-voltage capacitors have relatively small capacitance?

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Figure 1: Ferroelectric properties of barium titanate (BaTiO3). (Left) Above 120° C the structure of the BaTiO3 crystal is cubic, and there is no net polarization of charge; (right) below 120° C the structure changes to tetragonal, shifting the relative positions of the ions and causing a concentration of positive and negative charges toward opposite ends of the crystal. — capacitor dielectric and piezoelectric ceramics Figure 1: Ferroelectric properties of barium titanate (BaTiO₃). (Left) Above 120° C the structure of the BaTiO₃ crystal is cubic, and there is no net polarization of charge; (right) below 120° C the structure changes to tetragonal, shifting the relative positions of the ions and causing a concentration of positive and negative charges toward opposite ends of the crystal.

High-voltage capacitors generally possess relatively small capacitance because of the physical constraints required to prevent electrical breakdown. To store charge safely at high voltages, the electric field within the capacitor must be managed carefully; if the field becomes too intense, the insulating material will fail, leading to a discharge.1

While capacitance can be increased by expanding the electrode surface area or reducing the gap between electrodes, these adjustments are limited in high-voltage applications. A smaller gap increases the risk of electric breakdown, necessitating a design trade-off where the capacity is kept lower to ensure the device can withstand the required voltage without failing. In contrast, capacitors designed for low-voltage use can utilize thinner gaps or different materials to achieve much higher capacitance in a compact form.1