Why is this happening? By default, photo and video content are stored in their original high-resolution version. This results in taking up a lot of space on your iPhone, iPad, and iPod touch. 

What do I do? To solve the problem, go to Settings > Your Name > Photos, turn off iCloud Photos, and tap Delete From iPhone. Do not worry, the original high-resolution images will remain in iCloud. The System storage will return to normal after this.

Now, you may enjoy additional free space with your iOS device :)