LighterMac

Why is System Data so big on my Mac?

The short answer

System Data is everything macOS can't put in a named category. On most Macs that means app caches, browser data, chat app files, logs, developer folders and container disks like Docker's, plus macOS's own working files. It gets big because most of it is never trimmed. Nothing is broken; nobody is cleaning it.

What's usually inside

WhatTypical sizeSafe to clear?
Browser caches (Safari, Chrome, Firefox)2–15 GBYes. Rebuilt as you browse.
Chat app files (Slack, Teams, WhatsApp, Discord)1–12 GBYes. Downloaded again when you open the chat.
App caches in ~/Library/Caches2–10 GBYes, with the app closed.
Xcode build folders (DerivedData)5–80 GBYes. The next build is slower once.
Docker's disk image10–60 GBOnly from inside Docker. It holds your containers.
Browser profiles1–10 GBNo. Your logins and history.
Time Machine local snapshots0–100+ GBmacOS removes them itself when space runs low.

Check it yourself, without any app

  1. Restart your Mac. It clears memory swap and temporary files. System Data often drops a few gigabytes on its own.
  2. Look at your Library folder. In Finder, press ⇧⌘G, type ~/Library and press Return. Caches, Containers and Application Support are where most of it lives.
  3. See the biggest folders. This only reads, it changes nothing: 
    du -sh ~/Library/* 2>/dev/null | sort -h | tail -15
  4. Check for local snapshots. 
    tmutil listlocalsnapshots /
    If the list is long, Time Machine is holding space. macOS gives it back when something needs room.

Don't delete whole folders in Library. Application Support holds real data: your browser logins, app settings and chat history. Clear caches, not data.