imovie video shows up as small square

Hi - I took a few videos on my iphone 5 in portrait view (size is 1080 x 1920) and made a movie in imovie. When I export it to facebook it shows up as a small square with black space around all 4 sides, and I can't double click to make it larger. I tried uploading to facebook from my computer and my iphone and it didn't make a difference. When I shared the video from imovie via email and file (and saved it to my desktop them texted it to myself) it showed up on my iphone as the small square but if I double clicked on it it became full screen. There are a few different size options when sharing and I tried all of them. I did also upload to vimeo and youtube just to try and they come up fine. . with the black space on the 2 sides (but not top and bottom). I then tried accessing vimeo from my iphone and downloading the video, but the same thing happened with the small square.

I'd love to be able to share my movie via text message and facebook. . with it being full screen (when viewed on the iphone), or with black space on the sides but not top and bottom. Is this possible? Please help!

PS - sorry if this doesn't make sense, I'm so confused but I tried to explain it the best I could!