Two kinds of data

  • Cache: temporary media that can be re-fetched from the cloud, safe to delete;
  • Chat data: locally kept messages and files, deleting really removes them.

Cleanup

  1. Settings → Data and Storage → Storage Usage;
  2. Inspect totals or per-chat figures and tap Clear;
  3. Clear photos, videos, files and music independently.

Set and forget

Use Media Retention to keep viewed media for three days or a week; older items disappear automatically. Keep important files in Saved Messages, where they live in the cloud rather than on your device.