iCloud & iCloud Photos · iCloud Photos and the Photos library
What happens to iCloud Photos when my Mac's disk is almost full?
Short answer: With Optimize Mac Storage, Photos keeps smaller versions as space runs low. With Download Originals it needs room for every original, so downloads can stop on a full disk. Switch to Optimize, move the library, or free space elsewhere.
With Optimize Mac Storage, Photos swaps full originals for smaller versions as space runs low, so the library shrinks on its own over time. With Download Originals, Photos needs room for every original; when the disk is nearly full, new photos may stop downloading and Photos warns you.
If space is tight, switch to Optimize Mac Storage in Photos → Settings → iCloud, or move the library to an external drive. Freeing space elsewhere on the Mac helps too — see how to reduce Photos' space on the Mac.