If you have little space on your Mac, you can free up a lot of space by moving your Photos library to a remote drive. To do this:
Make sure you have backups of your photos. (Which, by the way, you should do anyway.)
How to move your photo library
1. Exit images.
2. Copy the image library from your image folder to an external disk.
3. Hold down the Option key (or all) and launch Photos. Select “Other Library” from the resulting menu.
4. Navigate to the new location (remote drive) you selected.
You’ll get a message that says, “This library contains items that need to be downloaded from the iCloud Photo Library.”
Click “Delete Incomplete Items.”
7. In the Photos app, go to “Settings” and under “General” click “Use as system photo library.”
8. Then go to “System Preferences> Apple ID> iCloud.” Make sure there is a check of Photos if you want the app to use iCloud for storage (and you probably do).
9. Test the library. When you are sure that everything is fine, you can throw it on your internal HD to free up space.