I would remove the app and reinstall it too but i'de first check what file would take up 21GB of storage.
Just to make sure it's really the app or if it's an app file.
e.g all photos you receive in whatsapp wether you want this or not are stored on your android phone storage. (not the sd card)
If you check the settings, android will say Whatsapp is using x amount of space but when you look into detail it's the media that's taking up this space.
If you can't be bothered just reinstall the app. 😃
Cheers,
Ben