You're overthinking this. The fixture wires, even if #18, are large enough to open the OCPD if there is a fault. Nothing will burn up, if this were an issue the NEC wouldn't allow a #18 fixture wire on a 20 amp circuit. Nothing wrong with a 15 amp duplex on a 20 amp circuit either.
I know. I overthink everything, haha. Sometimes it works to my benefit. Other times it throws me into an anxious tail spin.
Just humor me for a second, because I like to question everything.
Question #1) If the #18's are large enough to open the OCPD during a fault, that is to say, large enough to briefly handle an amount of current that exceeds the rating of the OCPD, why are they not considered large enough to handle the branch circuit current from say the switch to a single fixture? Obviously it's different if that branch circuit section is ahead of multiple loads.
Question #2) Isn't there a type of fault that doesn't pull enough current to trip the breaker? I want to say "partial short," but I'm not sure if that's the correct terminology. Perhaps I'm wrong and confusing this with the idea of a fault occurring on an improperly bonded/grounded raceway.
I know per code there's nothing wrong with a 15A receptacle on a 20A circuit, I'm just saying it bothers my OCD. My mind just conjures up the image of someone plugging in extensions to the point of pulling say 18A and burning up the receptacle.
I think the same of pulling a #14 switch leg from a switch fed by a 20A branch circuit and 20A OCPD. I know I'll catch flak for this, but if the OCPD is 20A, I typically pull #12's all the way to the fixture.
And before you ask, yes.. tap conductors bug me, lol.