If the wire diameters seem to small to you, here is a calculation:
Let's assume that your speakers have a resistance of 2 Ohm and your amp has
an output of 1600 Watts (that's the most demanding scenario in the table above).
Calculation of maximum current:
I = square root(P/R) = square root(1600 W / 2 =ohm) = square root(800 W/Ohm)
= 18 A
According to the DIN table below a wire of 2.5 mm2 diameter is almost sufficient;
however, voltage drop and damping also has to be considered.
A copper wire of 8.5 metre length (to be precise it's length is 17 metres,
there and back) with 4 mm2 diameter has a resistance of 0.048 Ohm. This means
you are giving away almost 2.5 % power and the damping factor is 40.