WhatsApp is the culprit in this situation.
Most of your WhatsApp data is not being stored in iCloud, as shown in your second screenshot, where WhatsApp is using 40.8 MB of data in iCloud versus 6.3 GB on your device.
WhatsApp does not delete data even when it's backed up to iCloud. Conversely, deleting chats from WhatsApp will remove them from the backup in iCloud. Backups of WhatsApp are only intended to save what's currently present in the app.
You can determine what chats are taking up the most space by opening WhatsApp, going to the Settings tab, tapping on Account, tapping on Storage Usage, and switching to the Size tab at the bottom. This will sort chats by size, largest at the top. Tapping on a chat will result in a list of usage by category - see screenshot below for an example. (Note I removed phone numbers/contact names and images.)
You can export chats, optionally including media, by opening a chat as normal, tapping on the contact name or group name at the top, then tapping Export Chat. It may take some time for it to create the chat archive. Once it does, a share sheet will appear and you can choose whether to email it, message it, or use other apps installed that can interact with .zip files (such as Dropbox).
Once you have exported it and saved it somewhere, it's then safe to delete the chat itself in WhatsApp. This will cause the chat and any attached media to be removed and you should see a reduction in the storage usage in the Settings app.