@hideki No file manager by default reason 1.
Three typical causes:
1. thumbnail.dat: thumbnails of every image and video ever, grows larger than your images because it doesn't clean after itself.
2. Browser downloads
3. Underreported whatsapp space cons.

@hideki@gameliberty.club I use this -
https://play.google.com/store/apps/details?id=com.google.android.diskusage