I have these two ceramic capacitors shown in the image below:
It seems to me that both of them have 2.2nF capacitance and the first one has 400V rated voltage while the other one has 1000V rated voltage.
I wonder if there is any difference between them (except for rated voltage) and in case they are the same why is the first one twice as big (although it has smaller rated voltage)?
The capacitor with the 250V AC rating is an X and Y rated capacitor and is capable of being applied across the incoming AC in your house. It's bigger because its reliability is much better i.e. it won't form a short circuit if it fails. Here's the idea behind them: -
As you can see a Y rated capacitor is designed to be between line and earth thus a properly designed capacitor will not short line to earth (electrocution possibility). The X rated capacitor can be put across live and neutral and does not need a fuse i.e. it does not pose a fire hazard because it fails safe = open circuit.
The capacitor on the left is therefore bigger because it is more reliable in these scenarios.
