This hack (found here) is an interesting piece of outsider art (insofar as any Metroid 1 hack could, conversely, be considered "insider art"). It's a fascinating example of an extremely talented person reinventing the wheel simply because they were unaware of the state of the art.
In short, my best guess is that the author:
- Was unaware of the existence of the current best Metroid 1 editor, Editroid.
- Was therefore unaware that Editroid could expand roms automatically, making space a non-issue.
- Opted to completely rehaul a significant portion of the game engine to implement some offhand suggestions that Snowbro offered decades ago (in the help files of MetEdit) to make the level format just a smidge more efficient.
- Thus made the hack incompatible with any existing tooling.
- Made a (nearly) complete hack regardless.
Dudes rock (genuinely).
Now, one cool thing about this hack (compared to other M1 hacks) is that it cleverly wraps the areas around the 32x32 world grid to create a novel world shape (which required a couple changes to the engine to avoid Some Jank). You can see this above where the first map above is the game's internal representation, while the second map is what it looks like unraveled. (My favorite detail is that while the Varia Suit and Ridley are technically right next to each other, they are in practice the entire length of the world apart.)
As far as the hack itself: it's okay. The amount of new screens the author could add seems to have been balanced out by how much larger they made the game world, so the end result is (depending on the area) about as repetitive as the original. However, if you're one to map things out for yourself, you might enjoy it (sorry for the spoilers though).

