Well I think you all know what the OP means. You cannot if you leave it on a 209 amp overcurrent protective device.
I agree that the reason the switch is allowed to be on a 20 amp circuit mostly because the load on the circuit is generally never that high and if it is then a 20 amp switch can be employed. However if the wire is 14 awg then that may not be able to be changed and the equipment grounding conductor would be smaller than required which could , in some situations , create a large impedance that may not trip a breaker in a ground fault or short circuit situation.
I disagree, the trip curves of a 20 amp breaker do not go up to the point that the impedance of a 14 gauge wire causes the EGC to melt but not on a 12 gauge wire.
In fact 4 scenarios exist where an undersized EGC still provides ground fault protection:
1. In older NECs the EGCs were permitted to be substantially smaller for 15,20 and 30amp circuits.
2. In Canada a smaller EGC is permitted where a 12 gauge 20amp circuit can have a 14 guage ground.
3. Circuits over 30amps can have reduced size EGCs like a 60amp #6 subpanel with #10 ground.
4. Breakers for motors and AC units can be severally oversized in relation to the wire to allow for inrush. I could have an AC with an MCA of 12 amps mx breaker size 40amps and put #14 on a 40amp double pole breaker which usually have higher magnetic trip curves that single pole breakers.