iMovie for iOS: Why are videos shared through the Camera Roll lower resolution?
You can share your finished iMovie for iOS video from the Camera Roll via email, iMessage, MMS, and to YouTube.
When sharing your finished iMovie video via email, iMessage, or MMS (iPhone 4 or later), the Camera Roll scales down and compresses the video in order to improve upload performance. If you exported your movie from iMovie in HD resolution when sharing to YouTube, you can select whether to share a standard definition or HD version of the video.
Regardless of how you share your finished movie, the original full-resolution version remains in the Camera Roll, and you can transfer this version back to your computer when you connect your device. To do this, follow the guidelines in Copying personal photos and videos from iPhone, iPad, or iPod touch to your computer.