Why is the capacitor large in withstand voltage and small in capacity?

Mainly, the unit "farad" of capacitance has a larger size.

Moreover, the volume of common capacitors is fixed, and the withstand voltage is inversely proportional to the capacity. With large capacity, the compression resistance is low, and vice versa.

Of course, there are also high-voltage and large-capacity ones, such as those in capacitor cabinets.