Negative cable size matters
“ Kirchhoff's current law (1st Law) states that current flowing into a node (or a junction) must be equal to current flowing out of it.”
The negative cable will move the same current as the positive cable unless there is measurable surface leakage from the chassis over the battery body towards the negative terminal.
For all intents and purposes that leakage can be assumed 0, but we know it happens a tiny amount.
-s
