According to this support document from Apple, iMovie for iPhone downsamples video—even HD video—before sharing it via YouTube, MMS, email, or MobileMe. The maximum resolution is 568 x 320, which provides better performance when uploading.
So, to get the best quality output, first export the video to the Camera Roll, and then import it to your computer using an application such as Image Capture or iPhoto.
Published by Jeff Carlson
Author, photographer, and late-nighter Jeff Carlson writes for outlets such as Popular Photography, DPReview, Creative Pro, and others, and is a contributing editor for TidBITS. He is the author of Take Control of Your Digital Photos; The Photographer's Guide to Luminar AI, Take Control of Apple Watch, and Take Control of Your Digital Storage, among many other titles.
View all posts by Jeff Carlson