The big advantage of ArangoDB with respect to memory/disk usage is that despite the Schema-less-ness, the database automatically recognises common "shapes" of the documents in a collection and thus usually does not have to store all attribute names many times. In addition, the possibility of transactions makes it less necessary to keep many old revisions of documents, in comparison to for example MongoDB.

I'd be more interested in a document store that was historical and didn't mutate.

Failing that, I'll just keep using RethinkDB for this sort of thing.

What do you think is the primary cause of excessive space usage in MongoDB?