The Cloud Overflow
macOS is deeply integrated with Apple’s iCloud ecosystem. By default, when you set up a new Mac, the operating system assumes you want a unified experience across all your devices. Therefore, the native Photos app is configured to automatically sync every single picture you import or take across your entire iCloud account. If you shoot a massive RAW photo library on your DSLR and dump it onto your Mac for local editing, macOS will immediately attempt to upload those massive files to iCloud, rapidly consuming your cloud storage quota and clogging your local network bandwidth.
Isolating the Photo Library
To preserve your cloud storage space and stop macOS from automatically syncing your local Photos library to iCloud, you must disconnect the Photos app from your Apple ID.
Open the Photos application on your Mac. Navigate to the top menu bar, click on Photos, and select Settings (or “Preferences” on older versions). A new window will appear; click on the iCloud tab at the top. You will see a master checkbox labelled iCloud Photos. If this box is checked, your Mac is actively syncing everything to the cloud. Click the box to uncheck it. A warning prompt may appear asking if you want to download a copy of your iCloud photos to your Mac or remove them from the Mac (they will remain safe in the cloud). Choose the option that best suits your local storage needs. From now on, the Photos app acts strictly as a local, offline gallery.