iMovie help

[Platyrrhine]

Hi, I recorded a video on my iphone 6 and just wanted to add music/sound effects. I was able to do this no problem from within iMovie. The problem I'm having is saving the video back to it's original size (full screen). When saving/exporting from iMovie back to the photo album it gives me different quality options (720, 1080p, etc). I've tried all of them however when I go back to the photo album the saved video is small/cropped and not full screen. I'm assuming it's changing the aspect ratio or something, but I'm unsure how I can simply save the video back to it's original specs and view it full screen. I'd prefer not to have to transfer it to a computer if possible. Any suggestions/help would be very much appreciated. Thanks!