The iPhone has a great camera, but it’s also a grade-A space hog. The nuclear option: Triple-check that your photo library is fully backed up on the cloud of your choice (iCloud, Dropbox, Google Photos, but really honestly just Google Photos), and/or a hard drive, and delete them all. Easy! I know it feels weird, but you were going to have to do it when you got a new phone anyway. Might as well cleanse in stages.
For those who favor a little more finesse, there are a few settings you can tweak that will minimize your gigabyte consumption along the way.
First up: If you have HDR on, which you might as well, your iPhone is storing two versions of every photo you take. One is HDRified (a few shots taken at different exposures at the same time, combined to make one super-picture); the other is the original. Wasteful! In Settings, go to Photos & Camera, then Keep Normal Photo, and unselect.
Next, if you’re not using Live Photos (or if you don’t know what Live Photos are), turn them off. With the camera app open, look for the icon with three concentric circles at the top of the screen. If it’s yellow, tap it. All of a sudden, you’re not taking mini videos every time you just wanted a still life.
And finally, speaking of videos, do you watch yours anywhere other than on your phone? If the answer is no, you should take the quality down a notch. Go back to that Photos & Camera setting and tap Record Video. The default is 1080p at 30 fps. You want 720p at 30 fps. It takes up less than half the space, and you won’t notice the difference on a smaller screen. Honestly, it’ll look acceptable on a TV, too, if it ever comes to that.