[NobodyNada]

> Super Metroid has a surprising amount of filler scattered all over.

It only really looks scattered because the visualizer uses a space-filling curve instead of laying out the bytes of the file linearly. If you change the curve to "scan" instead of "cluster", it becomes a little easier to see what the filler is: it's the unused space at the end of each ROM bank.

(As someone who's spent a lot of time in that particular binary, I think the use of the space-filling curve obscures a lot of details that would be a lot easier to spot otherwise -- such as the structure of the ROM banks, the location and layout of various pieces of compressed and uncompressed data, and repeated structures like a couple KB of common routines that are duplicated at the beginning of every bank containing enemy code.)