Well, not quite. Rust eats into iron because oxidised iron is larger and much more brittle than unoxidised iron, physically ripping itself out of place.
Many oxides arent that much larger than their base metals and form a nice patina protecting the metal underneath, like in aluminium.
Other oxides destroy the structural integrity of the metal and eat into it, forming corrosion. Rust is just corrosion specific to iron.
That because the big files right now are the OS. Just deleting system32 isn't a good idea, but moving to a more efficient system is difficult. So we do the easy thing and delete old PDFs, and maybe some old games. But the system needs to be changed, and the sooner the better.