Why does iMovie change QuickTime Video's size I used QuickTime to record my iPhone 6 device video. Format of iPhone 6 QuickTime video on desktop H.264, 750x1334. Size= 406x722. Then I import this QuickTime video into iMovie. When I export the video from iMovie back to desktop the format is now H.264, 1920x1080 and size= 1280x720.

Why does iMovie change the size? I want the size to always stay exactly the same as what I recorded on the iPhone 6 device into QuickTime NOT the size iMovie exports back to desktop. 

How do I make iMovie keep the size of the original QuickTime iPhone 6 video?
iMovie only generates video at specific resolutions. It's intended for simple use-cases, and so has a really cut-down feature set for video editing to make it harder for novice users to make mistakes. Unfortunately this means it often doesn't do what you want.

If you're only clipping the start/end of your video, you can just use Quicktime alone to achieve this. If you need to add titles or otherwise do some editing on your video, you can:


use iMovie, export to the maximum resolution available, then down-sample to your desired resolution; or
use some video editing software that actually supports custom output resolutions (Final Cut Pro, Adobe Premiere, Lightworks, etc).