First of all, I would maybe rethink the layout if you need longer wires. Usually, the combined length of the motor wires and ESC wires are long enough to reach without adding anything. Having motor wires that are too long can cause some issues - as with any high current application, the shorter the wires, the better.
Anyway, you can use a butt connector to lengthen the wires. I usually use
these (the yellow will fit 12GA, but not 10GA), cut the pastic sleeving off, and soldering the motor wire to one end and the ESC wire to the other. Then, I heatshrink the whole thing. The result is a pretty clean look without much of an unsightly bump.