It is easy to compress the videos recorded using your iPhone camera:
1. Download and install Video Slimmer App to your iPhone
2. Tap the + button to import video clips from your iPhone Camera Roll
3. Tap the Slim Now! button to start to compress the video files
4. Go to the Converted Video tab to check the output video files. There, you can also know about how much space you have saved by compressing the video.
Note 1: You can choose to merge several video clips into one single video file.
Note 2: If you want to discard unnecessary parts of any imported video, you can tap it to go and trim it. You can also directly trim video in iPhone camera roll before adding to the app.
Note 3: You can set video quality and video size for the output video before compressing. Just tap the More… tab and go to Settings to change the app settings.