If the IEC cable is short enough a smaller wire gauge really isn’t going to be an issue. My 50A oven has at most 14ga wire internally and hasn’t had a single problem. It’s such a short run that voltage drop across it is still less than 3% IF a full 50A was being pulled (unlikely). Now an oven is a manufactured and tested product and I don’t suggest running 50A down a 14ga wire. However a short jump from stringer to device in open air is unlikely to cause any issue. The breaker should trip before enough current flowed to cause a problem.
Your typical large heating element in an oven is about 2500 watts. At 240 volts, that's just over 10 amps. Each element is on its own run back to that main 50 amp source which is why they can safely use 14 gauge wire internally. Of course, if you had some sort of partial failure of an element that increased the current draw, there is risk that the internal cable could start on fire. That sort of failure is nearly impossible without modification, so it's considered acceptable. Even still, some devices will have additional circuit breakers or fuses on each element just in case.
While you won't see much voltage drop over a short piece of wire, voltage drop means heat is being generated in the wire. If you run 20 amps through a 6 foot piece of #18 wire, you would see a 3% voltage drop. That works out to 13 watts of heat dissipated per linear foot of wire.
In comparison, the same voltage drop on a piece of #12 would need a 25 foot cable to have a similar voltage drop, which works out to about 3 watts per foot.
At 20 amps, that #12 is going to get warm. If you coil up the #12, or bundle a whole bunch of cables with the same draw together so that the heat can't escape, it could eventually melt.
A piece of #18 at 20 amps will get VERY hot in open air. Hot enough that it will likely melt the internal wires and once those get soft, any movement of the wire can cause a short. ie, hot wire, someone bumps it, and that's when the short would happen. If it sits untouched, it could get melty, but not quite enough to short out if there's no stress on the internal components.
A breaker won't do anything to prevent this until it does short out, and then hopefully the breaker trips.
Just a recipe for disaster if you don't pay attention!
