its been stated that Vault space is a technical limitation
Namely the weapon comparison stat card data doesn't all fit into ram simultaneously when inspecting a proposed enlarged Vault
Here's a hilariously clever solution Bungie
Since the Cursor has a set max movement speed and is not truly random access, we can take advantage of the time delay to load different sections of the Vault into ram cache
Just reorganize the Vault weapon data into a binary space partition tree, then collision check cursor position against it to decide what to load
Easily cut the limit by half even in worst case
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Honestly, the current amount of space is fine. What they SHOULD do is just remove the different sections, that way all spaces can be taken up by any item.