Take a look at picture of 2 switches , both are for single gang only (except left one is dimmer switch). As per NEC single pole switch device has fill volume of 2. My question is, switch on the left is bigger than the one on right. Does it make difference in fill volume or both has same fill volume of 2.
Most switches are wired with 14 awg wire or 12 awg wire the box fill is based on the largest size wire connected to the device. 14 awg wire is 2 cu inches x 2 the fill allowance would be a total of 4 cu in for either switch with 12awg. 2.25 cu inch for 12 awg x 2 would be 4.5 cu inches with #12 wire. The sizes and cu inch are defined in table 314.16.B. There are other things that have fill allowance clamps X1 of the largest wire Grounds x1 of largest wire under clamp, hickeys x 1 of largest wire.