aspect ratio

Hi I am using iMovie to edit my clips and trying to export my movie to youtube, but upon upload, I find black bars side of my video when viewing in cinematic view. Based on my understanding, iMovie exports in default aspect ratio 16:9. When I check the file info, my .mov file has this...

Youtube recently updated their video player to include a dynamic aspect ratio for their theatre mode viewer. They however did not add the feature to the normal player. I'm glad they copied vimeo and added it, but creators are not aware about it and music videos and trailers routinely are still...