The factory reduces the shorter cable by at least 1 gauge on the cars I have measured. I prefer 2 sizes smaller.

There are a lot of naysayers who claim it's not important but when my starter wire grounded I didn't know it until I went to start the car. It saved my car.

Some people claim they are safer riding a motorcycle without a helmet too.

In your case since you are using 2/0 cable I would run nothing bigger than a 4 gauge short cable. Even a smaller ground won't help if both wires are so big they will not burn before the battery explodes. In an ideal wiring your ground wire should be the minimum size needed to start the car and the extra size of the positive is insurance against that worst case scenario.
After it's cranked you only need a 10 or 12 gauge positive wire for most cars to run on, so the 4 gauge is plenty if it cranks OK.