Comparable to standard house wire, which carries much more current for powering high power devices like vacuum cleaners etc, speaker wire seems to be unusually thick, and tends to be stranded, which correlates to even more efficient power transfer.
Why is this speaker wire so thick?
Even more than appliance wire, speaker wire needs to be low resistance. If not, it can interact with the inductance of the speakers and change the frequency response. Although the wide spread of premium low-resistance speaker wire seems partly unnecessary, since folks got along without it for several decades.
Speaker wire is also larger because it is "stranded" rather than solid. This allows for easier wire-runs. If you look into the skin-effect, you can lower resistance by having more skin surface area. This allows for runs of wire that have less of a statistical chance for creating their own filters, by lowering resistance, while having the mechanical ability to be routed in tighter spaces with sometimes odd angles. It typically uses thicker insulation to handle higher current. Those are pretty much what makes the cable "in whole" thicker.
As mentioned before, the resistance of the wire and the voice coil in the speaker create a frequency filter. The less resistance, the less overall distortion of the original signal while having enough mass to reduce resistive heating. (Which also increases resistance and compounding the filtering issue)
