That is the main feed wire from the pigtail to the magnets on the driver side. The axle wire is consistent with the size of the magnet wires. As I recall you can go big to small, but not small to big. I suppose you could make the 10 gauge a main feeder wire all the way, but that's not how it is from the factory. The magnet wires are only 18 ga. You would think it would be best to have the larger wire feeding each brake, but they seem to work fine as is.
I read another suggestion to install a terminal block and make the 10 gauge a home run wire to each brake. That way one failure won't knock out the whole system. Seems like a good idea. My guess is the brake controller would not identify a failure this way.