The short answer
Find your longest videos in Photos › Media Types › Videos. To keep a video but make it smaller, export a lower-resolution copy (1080p is plenty for a phone screen) with iMovie or the Shortcuts app, check it plays, then delete the original. In Settings › Camera › Record Video, choose a lower setting for everyday clips.
One minute of 4K video can take as much space as a hundred photos or more. A few long clips from a birthday or a concert can be most of your camera roll by size.
You don't have to choose between deleting them and keeping them full size. A phone screen shows 1080p perfectly well, and a 1080p copy of a 4K video is a fraction of the size.
Find the big ones
Open Photos, then Media Types › Videos. The length is shown on each thumbnail. Long videos are big videos, and anything recorded in 4K or at 60 frames per second is bigger still.
Delete the clips you'll never watch: the accidental recordings, the twelve-minute video of a stage you can barely see. For the ones you want to keep, make a smaller copy.
Make a smaller copy
With iMovie
iMovie is Apple's free video editor.
- Create a new movie project and add the video.
- Tap Done, then the share button, then Save Video.
- Choose a smaller resolution, such as 1080p or 720p, if iMovie asks.
- Play the new copy in Photos and check it looks right.
- Delete the original.
With the Shortcuts app
Shortcuts can re-encode a video too. Build a shortcut that selects a video, runs the Encode Media action with a smaller size, and saves the result to Photos.
What a copy loses
A video exported this way is a new file. It usually gets today's date, so it shows up at the end of your camera roll instead of where it belongs, and it may lose its location and albums. Check before you delete the original. A dedicated shrink tool that carries those details over saves you fixing it by hand.
Trim instead
If only part of a video matters, trim it. Open the video, tap Edit, drag the ends of the timeline, and choose Save Video as New Clip. Then delete the long original. If you only tap Save Video, Photos keeps the full original underneath so you can undo the trim, which saves little or no space.
Record smaller videos from now on
Open Settings › Camera › Record Video. The screen shows roughly how much space a minute of video takes at each setting.
- 1080p at 30 fps is plenty for everyday clips you'll watch on a phone.
- 4K is worth it for the things you might want to see on a big screen one day.
- In Settings › Camera › Formats, High Efficiency records smaller files than Most Compatible. It's the default on recent iPhones.
Your choice stays until you change it, so you can switch to 4K for the big moments and back afterwards.