Well, to be fair that's what I was thinking about, but if it's better to have two wires of the same size, I guess I'll go for a bigger one.
Update on some tests I did with a big /00 positive wire and a smaller section negative one :
when car is warm she struggles to start, I can see a huge tension draw on the voltmeter (goes from 12V to less than 10V), battery charges but not so much, a second symptom of this issue I guess...
