Why is my phone’s storage space decreasing even with no new apps or files?

1. Check and view the storage usage and identify background activities that take up storage space.
2. Clear cached data of apps (Settings > Apps > [app] > Storage > Clear Cache).
3. Uninstall unused apps.
4. Move photos, videos, documents, and other large files into an external storage such as a microSD card.
5. Delete unnecessary files and spam messages.