Alright, that was hard. Particularly without prior knowledge of what the whole map looks like. I'm surprised I've made it on the first try.
On a technical side, I really don't understand 600-something MB build size. Is this an Unreal Engine thing?
Well, I don't want to blame UE5 completely but it's definitely easier to make build (game package) larger than smaller.
I made a short test and package empty 3rd person tamplate without assets. It takes like 270mb.
Hah, thanks for the playing!