Whatsapp storage issue
The storage usage which you saw in the device settings page (App info), includes cached data too. Click on the Storage in the App info page of WhatsApp. There you will find a Clear cache button. Click on that.


Side note: WhatsApp's data storage information only shows the storage usage by multimedia and messages. Besides those things, WhatsApp also stores it's settings, multimedia that you have shared with others and downloaded statuses. So, you cannot rely much on the information which is shown as Storage usage in WhatsApp. If you want to know more about the real storage usage, open up a file manager and browse the WhatsApp folder inside the primary storage. See these screenshots and note the differences (114 MB vs 12 MB):