Cable size for 2 battery banks

In his video for installation of 2 EG4 6500 EX inverters Will Prowse has a 4/0 cable feeding both positive and negative buss bars from one battery bank. For the second battery bank he drops down to 2/0 I’m not sure why. Shouldn’t the second bank have 4/0 as well?

It would depend on how everything is connected.
Each bank will take half of the current in parallel.
So, a short distance of 2/0 is more than capable of carrying half of what 4/0 can.