Why your Mac keeps filling up on its own, and how to stop it

Why storage grows when you change nothing

macOS and the apps on it write files constantly without asking: a snapshot every hour before a backup, a cache for every web page, a copy of every photo you open in Messages. Most of this lands in System Data (older Macs call it Other), a bucket that Storage settings cannot explain. That is why the number keeps climbing while your Documents folder stays the same.

The good news: almost all of it is either rebuilt automatically or is a copy of something you still have elsewhere, so it can go without loss.

The bad news: you have to know where to look, because Finder hides most of these folders.

Track down the culprit

1. Open Apple menu › System Settings › General › Storage and note the size of System Data. Come back a day later and compare. If it moved by gigabytes, the culprit is one of the six below.
2. In Finder press Shift+Cmd+G, type ~/Library and press Enter. Switch to list view, sort by size (View › Show View Options › Calculate all sizes). Caches, Application Support and Containers are usually the top three.
3. In Terminal, tmutil listlocalsnapshots / lists Time Machine snapshots. More than two or three means they are holding space.

Tidymint's Space Lens page draws every folder as a bubble sized by real usage, with plain-language notes on the common ones, and its Statistics page charts free space day by day (7, 30 or 90 days), so you can see the day the drop started instead of writing numbers down. It is free and open source.

Time Machine local snapshots

• What it is: when automatic backups are on, Time Machine takes a local snapshot of your startup disk about every hour and keeps each for up to 24 hours, plus one of your last successful backup and one before each macOS update. macOS removes them when it needs the space, and Finder does not show them.
• How to tell: tmutil listlocalsnapshots / shows several dates, or Storage settings lists "Time Machine" or "Backups" as a big item.
• Fix: usually nothing, because macOS frees this space on its own when needed. To clear them now the way Apple describes: turn off Back Up Automatically in Time Machine settings, wait a few minutes, then turn it back on. Terminal users can also run tmutil thinlocalsnapshots / 999999999999 4.

Caches that rebuild themselves

• What it is: ~/Library/Caches and the per-app folders in ~/Library/Application Support. Browsers, Slack, Discord, Spotify, Zalo, Notion and Electron apps in general are the usual suspects; a browser alone can hold 2 to 5 GB.
• How to tell: the folders named after an app or a bundle id (com.google.Chrome, com.spotify.client) are large and their contents are days old.
• Fix: quit the app, move its cache folder to the Trash, reopen the app. It rebuilds what it needs. Do not touch Database, .db, .sqlite or Preferences; that is where logins and messages live. Some apps have their own button: Chrome › Clear browsing data, Spotify › Settings › Storage, Zalo › Settings › Data management.

iPhone and iPad backups

• What it is: every time you back up a device to the Mac, a full copy goes to ~/Library/Application Support/MobileSync/Backup. A phone backup is 10 to 100 GB, and old devices leave their backups behind forever.
• How to tell: Storage settings shows "iOS Files" or that folder is huge.
• Fix: in Finder select your device in the sidebar, click Manage Backups, delete backups of devices you no longer own. Keep the newest one for the phone you use.

Mail and Messages attachments

• What it is: Mail downloads every attachment of every account into ~/Library/Mail; Messages keeps every photo and video anyone sent you in ~/Library/Messages/Attachments. Both grow by the day and never shrink on their own.
• Fix: Mail › Settings › Accounts › your account › Download Attachments: set to Recent or None, then Mailbox › Erase Deleted Items. For Messages, in Storage settings open Messages and delete large attachments from there, or set Settings › General › Keep Messages to one year (note: this deletes whole conversations older than a year, not only the files). Cleaner apps should leave these folders alone, and Tidymint does.

Developer tools

• What it is: Xcode's DerivedData, iOS simulators and old device support files (~/Library/Developer, can reach tens of GB); Docker Desktop's single disk image that only ever grows; node_modules folders in every project; Homebrew, npm and pip caches.
• Fix: Xcode › Settings › Locations › DerivedData, delete the folder; xcrun simctl delete unavailable; Docker › Settings › Resources › Disk image size, then docker system prune; npm cache clean --force, brew cleanup. Old node_modules in projects you have not opened for months can go; npm install brings them back.

AI models and image generators

• What it is: Ollama stores every model you have ever pulled in ~/.ollama (4 to 40 GB each); Hugging Face caches downloads in ~/.cache/huggingface; Stable Diffusion front ends keep checkpoints and every generated image. Few of these show up in Storage settings under a name you would recognise.
• Fix: ollama list then ollama rm <model> for models you no longer use; delete old entries in the Hugging Face cache; move generated images to an external drive. These are the folders that surprise people most, so they are called out by name in Tidymint's Space Lens.

Free up the space with Tidymint

The six places above are exactly what Tidymint scans: Clean Now groups caches, logs and old installers with a safety label on each group and cleans only when you click; Space Lens draws snapshots, iPhone backups, Xcode, Docker and Ollama folders as bubbles with plain-language notes; Clean by App shows what Zalo, Chrome and CapCut keep, with Zalo media sorted by month. Everything goes to the Trash, with Undo. Free and open source.

Keep it from coming back

• Restart once a week. macOS purges temporary files and purgeable space on restart.
• Empty the Trash. Files in the Trash still count. Finder › Settings › Advanced › Remove items from the Trash after 30 days does it for you.
• Set limits where the app allows it: Mail attachments to Recent, Messages to one year, Docker disk image size, Spotify cache size.
• Check once a month. Five minutes in a disk map beats a panic when the "disk almost full" warning appears.

Cleaned a bit too much? No worries: Tidymint only moves leftovers to the Trash, so everything can be undone if you change your mind.