Will a breaker protect its circuit if there's a larger wire upstream?
The breaker doesn't know about the heating of the wire.
The breaker has its own internal &quot;specimen wire&quot; that is in series with the wall wire.  It is measuring how fast its &quot;specimen wire&quot; heats up, and will trip at the appropriate time.
Literally, the &quot;specimen wire&quot; is a bi-metal strip, which provides both the sense and the muscle to pull the hammer on the breaker trip.