The Photos library is deceptively heavy: a 60 GB library may hold only 35 GB of media you would miss. The rest is duplicates from imports, burst-series you never picked, edited versions stacked on originals, and a thumbnails database that never shrinks on its own.
du -sh ~/Pictures/Photos\ Library.photoslibrary 2>/dev/nullCompare that to what you would actually keep. The gap is your reclaimable target.
Recent macOS has a built-in Duplicates album (Photos sidebar → Duplicates) — merge them in pairs; it keeps the highest-quality copy. Bursts: open a burst, tap "Select…", keep the one or two frames you want, and discard the rest — each burst can hide 10-20 unviewed frames.
Sort media by size: in Photos use the "Media" filter and review 4K videos — a single minute of 4K60 is ~400 MB. Screen recordings nobody re-watches and slow-motion experiments are the usual giants. Delete from Photos (not Finder), then empty the Recently Deleted album — items sit there for 30 days still occupying space unless you empty it manually.
Never dig inside the .photoslibrary package with Finder or rm — that is how libraries get corrupted. Everything legitimate happens inside the app or via File → Export. If iCloud Photos is on, deletions propagate to the cloud: export before deleting anything you might want.
Photos is one giant among many — System Data explained maps the rest, and the ten-second estimator ranks your personal suspects. For visual confirmation after the purge, DaisyDisk vs free treemaps is the short read.