Wiki

You can double the capacitance by doubling the surface area that the charge has to spread out in. But you can also do so by halving the thickness of the insulating layer, as this allows the charge on one side to more easily push charge out of the other. But a large enough voltage would destructively break through a very thin insulating layer. A capacitor therefore also has a voltage rating, being the highest voltage that it can safely withstand. This must on no account be exceeded.