Capture One catalogs keep previews of every photo, and they can outgrow the photos
Capture One stores previews and thumbnails inside each catalog or session. How to see the cache, choose a sensible preview size, and move catalogs off the Mac.
Capture One builds a preview for every image so browsing is fast. Those previews live inside the catalog, or inside a session's folder, and with large preview sizes and many thousands of images they add up to many GB.
find ~/Pictures ~/Documents -maxdepth 3 -iname '*.cocatalog' -exec du -sh {} + 2>/dev/null
find ~ -maxdepth 4 -type d -name 'CaptureOne' -exec du -sh {} + 2>/dev/nullA catalog is a package. Control-click it and choose Show Package Contents to see the Cache folder with previews and thumbnails.
Keep previews smaller
The preview size setting decides how large each preview is. A size near your screen's width is enough. Larger previews look no better on your display and take much more space. Changing it affects new previews; regenerate old ones if you want them smaller too.
Clearing the cache
Previews are rebuilt when needed. With Capture One quit, the preview cache inside a catalog or session can be cleared; the next browse is slower while it rebuilds. Your adjustments are stored separately and are not affected.
Referenced originals
If a catalog manages the originals inside itself, the catalog includes every RAW file. Referenced catalogs keep originals in folders you choose, which makes it easier to put them on an external drive.
Related: Lightroom previews, Mac storage for photographers.
Common questions
Where is the Capture One cache on a Mac?
Inside each catalog package, in a Cache folder, or in a CaptureOne folder inside a session.
Is it safe to delete Capture One previews?
Yes, with Capture One quit. Previews rebuild when needed, and your edits are stored separately.
Why is my Capture One catalog so large?
Large previews for every image, and possibly the original RAW files if the catalog manages them internally.