iPhone STORAGE MEMORY ALWAYS FULL: iPhone 6S+ 16GB was perfect at the beginning. Not anymore, why?

I have no social media apps at all. The only social media app I have is WhatsApp that I clean EVERY TWO MONTHS (with NO auto save media on Camera Roll). I have NO music (0), NO video (0), No pictures (o) on my iPhone 6S 16Go. I have NO games app AT ALL. My phone keeps saying that I have no memory. I can't even take a picture with my phone anymore. I always have to uninstall WhatsApp and reinstall it so I can use it.

QUESTION: Why iOS and SYSTEM DATA both keep my phone full ? It wasn't like that AT ALL when I first got my phone. The more I clean, the more it's full. My iMac starts to do the same the last two years.