The absolute value of resistance increases with the length. A 1,5 mm² wire suffers from an increase in resistance due to skin effect of ~10 % at 20 kHz.
20 cm of 1,5 mm² has ~2,2 mOhm at DC. That is 4,4 mOhm, if you add up the two wires for flow and return. 10 % of that means 0,44 mOhm more at 20 kHz than at 0 Hz.
Do the same calculation for 2 m of 1,5 mm² and you get 4,4 mOhm of difference. For 20 m it is 44 mOhm of difference.