I myself actually had no idea succinct data structures existed until last December , but then I found a paper that used them in the context of XML. Just to be clear: it's 120% of the original size; as it stands this library still uses more memory than the original document, just not a lot of overhead. Normal tree libraries, even if the tree is immutable, take a parent pointer, and a first child pointer and next and previous sibling pointers per node. Even though some nodes can be stored more compactly it does add up.

I suspect with the right FM-Index Xoz might be able to store huge documents in a smaller size than the original, but that's an experiment for the future.