I think, aside from the ends the only difference I see is the thickness. The brake cable looks to be about 1/8" and the throttle seems to be 1/16".
I replaced my brake cable with 1/8" wire cable sold at home depot.
Yup, brake cable is usually thicker because you put more stress on it when applying brakes. Just think of the stress needed to keep a mechanical caliper against the disc to stop. The throttle cable has almost no stress on it, so it can be thinner