Photos eating your Mac's storage? iCloud “Optimize” explained

If Photos is taking up space on your Mac, it's usually one folder doing the damage: your photo library. It holds every full-resolution original plus a pile of generated thumbnails and renders, and it grows quietly for years. iCloud Photos is supposed to help with a setting called "Optimize Mac Storage." Here's what that actually does, and why the disk can stay full anyway.

Where your Photos library lives

Your entire photo collection sits in a single package at:

~/Pictures/Photos Library.photoslibrary

That .photoslibrary is a bundle. Right-click it in Finder and choose Show Package Contents and you'll see what's inside: the full-resolution originals, plus the thumbnails, previews, and render caches Photos builds so scrolling and editing feel instant. The originals are the bulk of it, but the generated files aren't free either. On a library with years of photos and 4K video, the whole thing can run to hundreds of gigabytes.

Optimize Mac Storage vs Download Originals

If you use iCloud Photos, Photos gives you two ways to store that library locally. You pick one in Photos → Settings → iCloud:

• Download Originals to this Mac: keeps the full-size versions both on your Mac and in iCloud. Everything is local, so it uses the most disk.
• Optimize Mac Storage: keeps the full-resolution originals in iCloud and stores smaller versions on your Mac when space is limited. This is the one that frees space.

In Apple's own words, Optimize Mac Storage "stores smaller versions of your photos on your Mac when storage space is limited, and keeps the original, full-size photos in iCloud." So the full-res files live in the cloud, and your Mac keeps lighter copies it can swap back for the originals on demand.

One thing to state up front: this only works if your iCloud storage is big enough to hold the whole library. The free tier is 5 GB, so a real photo library means a paid iCloud+ plan.

Why Photos is taking up space on your Mac (even with Optimize on)

"Optimize" is not "empty." Even with it turned on, Photos keeps plenty local:

• Recently viewed and recently added photos stay at full resolution. The setting only downsizes older items, and only "when storage space is limited."
• Thumbnails, previews, and render caches for the entire library stay on disk regardless, so Photos can show you everything instantly.

And if you have Download Originals to this Mac selected instead, or you don't use iCloud Photos at all, then every original is local by definition, and a big library will fill the disk. That's the usual reason Photos still shows up as a giant chunk of storage even after you thought you'd offloaded it. Because a lot of this lands in the catch-all System Data bucket, it's easy to miss where the space actually went.

How to free the space (safely)

Start in the GUI; you rarely need the Terminal for this.

1. Turn on Optimize Mac Storage. Open Photos → Settings → iCloud, make sure iCloud Photos (Sync this Mac) is on, then select Optimize Mac Storage. macOS starts swapping local originals for smaller versions as space gets tight. This is the single biggest, safest win.

2. Empty "Recently Deleted." Photos keeps deleted items in the Recently Deleted album for up to 30 days before they're actually gone, so photos you "deleted" are still using space. Open Recently Deleted in the sidebar and delete them for good.

3. Review Duplicates. Recent versions of Photos detect duplicates for you. Open the Duplicates album in the sidebar (it only appears when Photos finds any) and merge them. Merging keeps the highest-quality version and reclaims the rest.

4. Move the library to an external drive. If you want every original local but not on your boot disk, quit Photos, drag Photos Library.photoslibrary from ~/Pictures onto an external drive, then hold Option while you double-click it there to reopen it, and set it as your System Photo Library in Photos → Settings → General. Verify it opens from the new location before you delete the original.

One caveat before you flip any iCloud switch: turning off iCloud Photos re-downloads your originals. If you had Optimize on, your local copies are the smaller versions, so before you disable syncing, Photos wants you to Download Originals to this Mac first, which pulls every full-res file back down and uses more space, not less. Turning iCloud Photos off is not a way to free space.

The honest short version

If your library is on iCloud, turn on Optimize Mac Storage and empty Recently Deleted. That's most of the win. If you want everything local, move the library to an external drive instead. Photos is often the single biggest real chunk of a full disk, right up there with old iOS backups; for the full order of operations, see how to free up space on your Mac.

Working this out by hand means guessing whether that giant library is full originals or already optimized, and hoping you dragged the right .photoslibrary to the right drive. Plumby maps where the space actually goes across your disk in plain terms: it shows the real on-disk size and exact path of your Photos library next to everything else filling the drive, so you can see what "Optimize" actually left behind. It reads and measures. It never touches your photos.