iPhone photos phantom storage (13 photos take up 13 gb)

I have a storage problem on my iPhone X. I have iCloud photos and a massive cloud library of ~700GB which used to be all squished into my 64Gb iPhone X, taking up around 22Gb. I recently wanted to free up space to do a WhatsApp backup, but that required 13Gb of free space which I didn’t have, so I decided to untoggle iCloud Photos and delete all my local photos except for my 13 most recent ones.
After hours of manually doing that, and emptying the recently deleted photos, I made sure that there was literally thirteen photos on my phone. But somehow Photos is still taking up 13Gb… What is going on???

My only possible explanation is that my Whatsapp takes up 12.5Gb, and the system is double counting the media on it. Could that be why? Then I’m in a dilemma because I need to free up another 5.5Gb according to WhatsApp to do a full backup