My video's export from iMovie in .mp4 - What version of iMovie are you using? Seems you are upscaling the video in that converter from 720p to 1080p. If the native file is 720p then export it at 720p. I think you'd really benefit from using HandBrake. I uploaded my HandBrake Tutorials to help people using any video platform. Your files will be even smaller and more optimised and I also cover an area which is often overlooked and that is audio in video files.
I do export it to 720 but it saves it in .mov and not mp4 to be honest the convertor I use does the trick but good thing about handbrake though.
I'm just uploading a video now about my experiences uploading to DTube and giving some tips. I think from looking at the size of your files you'll be able to shave off hundreds of megabytes from your files using HandBrake.
Thank you for sharing I will be sure to watch it for sure.