Mac storage basics · System Data and the storage categories
Why is the macOS category in Storage settings so big, and can I shrink it?
Short answer: It's the sealed, read-only macOS system and can't be trimmed. Shrink System Data, your apps and your own files instead.
The macOS category is the operating system itself — the sealed system volume with its apps, frameworks and fonts. Its size depends on the macOS version and isn't something you can trim; it's protected and read-only by design.
What you can control sits elsewhere: System Data, apps you installed, and your own files. Old Install macOS apps in your Applications folder are separate and can be deleted if you don't need them.