Why is 6S video grainy after iMovie edit, export to MBA?

1. I shot some video on my iPhone 6S, using the default video app, with default settings.
2. I edited the video(s) on my phone, using iMovie.
3. I mailed it to my MacBook Air using gmail.

The resulting 2 minute file is 13MB on my MBA disk (smaller than expected), and quite grainy - surprisingly low quality.

Does the iPhone just not shoot high-quality video, or did I mess up one of the steps above?

Any insight and advice appreciated - thanks.