Why macOS 15 Sequoia's 'System Data' Is So Big (and How to Trim It)

System Data on a fresh Sequoia install is around 12–18GB. On a Mac that’s been running Sequoia for six months without cleanup, it’s routinely 80GB or more. The OS hasn’t doubled in size — caches, snapshots, and Sequoia-specific new categories have piled up.

Here’s what’s actually in System Data on macOS 15 and how to bring it back to reasonable.

Why “System Data” is a black box

Apple uses “System Data” as a catch-all category for everything that doesn’t fit the other Storage labels: Apps, Documents, Photos, Mail, Music, Messages, iCloud Drive. That includes:

• Caches (system and per-app)
• Logs
• Time Machine local snapshots
• macOS install staging files
• Apple Intelligence model files (M1+)
• iPhone Mirroring caches (Sequoia-specific)
• Mail attachments
• Old iOS device backups
• Spotlight index
• Language assets

System Settings → General → Storage doesn’t break this down. You see a teal bar labeled “System Data” with no breakdown. That’s by design — Apple doesn’t want you deleting things you shouldn’t.

The result is a guessing game. Knowing the actual locations turns it into 30 minutes of cleanup.

Sequoia-specific contributors

A few categories are new in Sequoia or grew significantly:

iPhone Mirroring cache

~/Library/Group Containers/group.com.apple.iphonemirroring/

If you’ve ever opened iPhone Mirroring, this folder grows but doesn’t shrink. On a heavy iPhone user’s Mac, it can hit 8GB.

To clear: quit iPhone Mirroring entirely (Cmd+Q), then drag the Caches subfolder to Trash. macOS rebuilds what it needs.

Apple Intelligence model files

If you have Apple Intelligence enabled on an M-series Mac, the foundation models live in:

• /System/Library/AssetsV2/com_apple_MobileAsset_* — the model files themselves
• ~/Library/Caches/com.apple.AppleIntelligenceReport/ — diagnostic data
• ~/Library/Apple Intelligence/ — your generated content cache

Total can be 4–7GB. You shouldn’t manually delete the system model files; macOS will redownload them, wasting bandwidth. But the user-level cache is fair game.

If you don’t actually use Apple Intelligence, turning it off in System Settings → Apple Intelligence & Siri prevents further growth. Existing files stay until macOS purges them on its own schedule.

iCloud Drive caches

Sequoia uses ~/Library/Caches/com.apple.bird/ more aggressively than Sonoma did. If you have a lot of iCloud Drive content with “Optimize Mac Storage” off, this can grow quickly.

Game Mode performance logs

~/Library/Containers/com.apple.GameController/Data/Library/Caches/

Small per-game, but accumulates if you’ve played many titles.

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The big universal categories

Beyond Sequoia-specific, the same offenders that bloat System Data on every macOS version:

Time Machine local snapshots

Sequoia keeps these even when no Time Machine drive is connected. They can total 30–60GB.

Terminal:

tmutil listlocalsnapshots /

If you see entries, delete them all:

for snap in $(tmutil listlocalsnapshots / | awk -F. '{print $4}'); do tmutil deletelocalsnapshots $snap; done

macOS creates fresh ones as needed.

Old iOS device backups

~/Library/Application Support/MobileSync/Backup/

Each subfolder is a complete iPhone or iPad backup. Sequoia (like every macOS) doesn’t auto-rotate. If you’ve been backing up devices for years, 50–80GB here is normal.

Sort by date, keep the most recent backup of each device, delete the rest.

Mail attachments

~/Library/Mail/V10/MailData/<account>/Attachments/

Mail caches attachments locally even when they’re already on the IMAP server. Heavy email users routinely have 10–20GB here.

To prevent further growth: Mail → Settings → Accounts → Account Information → Download Attachments → set to “Recent” or “None”.

App caches

The biggest cache offenders on Sequoia:

• Xcode: ~/Library/Developer/Xcode/DerivedData/ (10–50GB if you build iOS apps)
• Chrome / Arc / Brave: ~/Library/Caches/Google/Chrome/, ~/Library/Caches/com.thebrowser.Browser/
• Slack: ~/Library/Application Support/Slack/Cache/
• Spotify: ~/Library/Caches/com.spotify.client/Data/
• Photos: ~/Pictures/Photos Library.photoslibrary/resources/streaming-thumbnails/
• VS Code: ~/Library/Application Support/Code/Cache/

Quit each app first, then drag its cache folder to Trash.

Old macOS installers

Check /Applications/ for “Install macOS Sequoia.app” left behind from updates. It’s 13GB. Drag to Trash if present.

Also check /Library/Updates/ for partial update files.

Logs

~/Library/Logs/ and /Library/Logs/. Usually small, but if you’ve had a misbehaving app dumping crash logs for months, they can total 1–3GB.

System logs in /private/var/log/ rotate automatically; don’t manually delete.

Step-by-step: trimming System Data

The order I’d recommend:

1. Update to the latest 15.x point release. Apple has improved storage handling in 15.1, 15.2, and 15.3.
2. Restart, then wait ten minutes idle. macOS runs cleanup tasks during idle time.
3. Empty Downloads (~/Downloads/).
4. Empty Trash (right-click each Trash in Dock).
5. Trim Time Machine local snapshots (Terminal command above).
6. Delete old iOS backups in ~/Library/Application Support/MobileSync/Backup/.
7. Clear iPhone Mirroring cache if you use it.
8. Quit and clear Slack, Spotify, Chrome caches (the usual heavy hitters).
9. Set Mail to download recent or no attachments, then clear Mail’s existing attachment cache.
10. Remove old macOS installers in /Applications/ if any.
11. Restart again, check Storage.

Realistic recovery: 40–80GB on a Mac that’s been running Sequoia for months without cleanup.

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What you shouldn’t touch

A few things that look like cache but aren’t:

• ~/Library/Containers/<app>/Data/ — the actual app data. Don’t delete unless you’re uninstalling.
• /Library/Application Support/Adobe/ — Creative Cloud installer cache; can be cleared but Adobe will need to redownload large amounts on next launch.
• /private/var/db/com.apple.xpc.launchd/ — system service state.
• /private/var/folders/*/T/ — temp files; macOS manages.
• Anything in /System/ — protected by SIP anyway, but don’t try.

If you’re unsure about a folder, sort it by date modified. If it’s actively being written to, it’s probably load-bearing.

Why System Data grows even when you delete files

Two reasons people see System Data not shrinking:

1. APFS snapshots. When you delete a file, the space isn’t immediately freed if a snapshot still references it. Time Machine local snapshots are the most common culprit. Trim them and free space appears.

2. Trash isn’t empty. macOS reports trashed files in the original category until Trash is emptied. Empty all Trashes.

Restarting after a big deletion also helps — macOS reconciles its storage view fully on boot.

A note on third-party tools

Several apps claim to dramatically reduce System Data. Most are doing the same things you can do manually:

• Clear caches
• Trim Time Machine snapshots
• Find old iOS backups
• Find duplicate files

The benefit of a tool is mainly time. Manual cleanup of all the locations above takes 30–60 minutes if you know exactly where to go. A good tool does it in 2–3 minutes.

If you trust a tool, use it. Sweep is one option (notarized by Apple, made for this exact use case). The category is competitive and the underlying work is the same.

Maintenance going forward

The two-minute monthly check on Sequoia:

1. Empty Downloads.
2. Empty Trash.
3. Trim Time Machine snapshots if not on a backup drive currently.
4. Glance at System Settings → General → Storage.

Twenty minutes once a month keeps System Data from creeping up to 80GB. The single biggest preventive habit is keeping Downloads under control — almost everyone’s biggest accidental contributor to bloat.