It's what they are housed in not really the wall. Romex wire covering traps heat to some extent is it enough to cause a problem? Probably not because the power demand is not there in a normal household so a #12 is suitable for 20 amps and a #14 is fine for 15. When you start bundling numerous current carrying wires in conduit then it becomes a problem. A good example is service drops the 2/0 power co over head wire will stand 700 amps (so I'm told never tested it!) but once it is connected to the house sheathed 200 amp wire must be 4/0 or even open wire in conduit.
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