Why do some capacitors have a dependency on capacitor size vs ESD?
There is good reason for a capacitor to be ESD sensitive. If you push enough charge into it, you will eventually exceed the dielectric strength and breakdown voltage, and the insulation will break down. So the ESD susceptibility is dependent on the capacitance and breakdown voltage. A high capacitance high breakdown voltage device will have low ESD susceptibility, but a low capacitance low voltage capacitor could be easily damaged by ESD.
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Keep in mind that ESD events are represented by the human body model which is typically 150pf and 330 ohms source impedance. The interesting part of an ESD event on a capacitor is that the smaller cap (150pf) is charging the larger cap on the board etc. through the equivalent 330 ohm source impedance. If you do some quick basic EE math (conservation of charge...CV=CV) you'll find that larger value capacitors will charge to smaller voltages even if the smaller cap was charged to 15,000 volts. So to supplement the comment, essentially only caps in the region of hundreds of Nanofarads or less will likely be susceptible even if they are only 50 or 100 volt caps. A .1 microfarad cap rated at only 50 volts for example should easily handle a 15Kv ESD event.
I agree that it is only small value low voltage capacitors that are likely to be ESD sensitive.
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