Why does iMovie change QuickTime Video's size
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).