Thicker wires have less resistance, so they can transfer more power without heating up. Heat = wasted energy. In theory thicker wires would give you more power, because less heat was created so you have more power to make the wheel accelerate. But soldering and connecting points increase resistance, if you can't manage to replace the entire phase wire, the point where you soldered the thick and thin wires together will make lots of resistance, because lead and tin in the soldering wire is a worse conductor than copper. I just finished replacing my phase wires, the stock wasn't that thin, and I soldered 10 awg cables to the stock phase wires, but even the 10 awg cables were warm, so you could imagine how hot those stock phase wires was. I had to increase the hole from 10,5 mm to 13 mm, but I was able to pull 7 AWG (!!!) wires through the axle. I hope that it makes difference in power end efficiency. I had to remove the sillicone from the wires and I applied heat shrink tubes so they are thinner at the motor. I hope that it won't melt, it says 125°C on it and 200°C on the silicone, so not much difference.
Btw I had to lengthen the axle to fit into my motorcycle fork, I was worried, tired so at AM 2:30 that's what came into my mind and I welded them together.
Anyway, why do my phase wires look burnt and why only 2?