By default, if you do screen recording in Mac, it is saved as QuickTime .mov format. If you want to convert to mp4 for more common format in the public and smaller file size, you can use ffmpeg to do it.
Install ffmpeg by Brew.
brew install ffmpeg
Then run command in terminal.
ffmpeg -i /path/to/input/file /path/to/output.mp4
That’s it. The file in 720p was about 694MB in mov but it is only 57.7MB after converted to mp4.