From now on, please all peeps label me as Charlie.....I wish nothing negative to "UH" if I do a FUBAR. I know him and fly his ships at times, I do not speak for him.

Whatever......while a 12ga wire "may" handle a lot more than 20A, for how long, what insulation (yes, I am sorta versed from PVC/MTW through Tefzel and above....;-)).
As stated before, big wire from battery to buss is great.....
Fuses are basically the resistance and should fail before wire becomes a "heating element/light bulb".,...
So, specking (SP) a low amp fuse can/will drop voltage to devices.

To others......do you want to use a 12ga jumper cable to get home?, or maybe 4O welding cable to handle the current?
High load WILL roast a small cable/wire. Short very high load may pass, not our issue.....we want to know the downside of very small fuses/breakers versus voltage supplied to devices....
My thought.....big wire, great connections, protection (fuse or breaker...), and have fun.