I've tried changing chest sizes using json files in config and I couldn't get the numbers to work well. I thought that maybe I mess up height or something but then I copied over the iron into copper and it did increase the container size but didn't adjust the UI. Trying the same using datapack resulted in the same outcome.

the UI is an image it isnt automatic as of right now they are a separate texture that needs to be changed or added, possibly in the future I will make it automatically generated but not right now