i've never used iMovie.
what are the dimensions of your video that is output from iMovie? the file size? What are you using to encode it for upload? or are you just uploading that file?
use something like mpeg streamclip to encode your final movie for web upload with these settings:
export to mp4, h.264 codec, unscaled (720p), then choose a bitrate (like 5 or 10 Mbps).
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