So, when I want to free up some space, first yesterday it said WhatsApp was worth 207MB, and then today it said it was only worth 166MB, What's happening here?!
We can't tell how you are freeing the storage. If you are using any third party app then it will free the caches of whatsapp. In which case no one can say how much storage is used by your whatsapp cache. Whatsapp it self deletes some caches when there is no need of it. So, if it's cache then it's totally normal.
Other than that you should look into Whatsapp's media folder where all your images,audio,video and docs are stored.