Why is 0V cable thinner?

[donkmeister]

I bought a towing electrics kit, and something that caught my eye is that the 0V / Ground / Earth cable is thinner than the 12V / Power cable. Neither is particularly chunky as this is just for the relay box, not a split charge relay.

I'm no electrical engineer, but I assumed that the return path would be carrying the same current as the power path (albeit over a shorter distance to the nearest earth stud) so this surprises me. Does anyone know if there's a reason why this is acceptable here?

Certainly when I had a quick look at amplifier wiring kits, these seem to use the same gauge of wire for 12v and 0v, also with a short earth cable.

[E-bmw]

Volt drop over distance dictates that a shorter cable does not need to be of the same CSA.