I did the maths on voltage drop between 35mm2, 50mm2 and 70mm2 some time ago. As the voltage is approx 14V, the current drawn by the bow2's can be quite high, cant remember what we measured at but its considerable when on load, then the volts drop of the cable could be a factor. I seem to have lost the scrap of paper I had it worked out on, but as some 70mm2 came up on the forum then I went for that. I will echo what Luke has said, 70mm2 is massive and doesnt like to bent so routing can be an issue in tight spaces.
Rich.