Most of the time you won't be on full power and when you are it is usually a short burst so apparently slightly undersized cables will be ok.
If you are planning on sustained periods of very high power then you will need bigger cables to reduce over heating and losses.
I'd say use what you have and monitor. If you need to you can always replace the shorter controller to motor cables with 70^2mm later.
As has been said, good quality connections are important.
I run 35^2mm on my tractor (48v, 400A), and I have some 70^2mm for my trike.
I might run the 70^2mm for the whole circuit, or just for the motor with 50^2mm for the pack.
Then again I also have some 95^2mm so I might run that for the motor with the 70^2mm for the pack. That would weigh a fair bit more though.