Not only sensor size but the ergonomics. I agree that in daylight my iPhone 6S is basically as good as my somewhat older Nikon, but the way you handle them both in order to take a photo really play into the final quality of the image.

The phone will always be like a tiny window to the world, while the camera is more of an extension of my eye. Might be a personal take, but for me it makes a lot of diference and it shows in my photos.