You could reduce the bitrate by exporting at the Custom setting under Quality. There will be a slider where you can reduce the bitrate.
Regarding format, you could shoot the video in a more compressed format, or choose Mp4 rather than pro res for iMovie export (which you already have done). The problem with shooting in a more compressed format is that iMovie unpacks video clips for editing and could export the final movie in a less compressed (thus larger file size) format than you originally imported.
My comment on file size meant to record a shorter movie or trim the movie so that it is smaller. A movie can always be made smaller.
Regarding compression, as you probably know the higher the compression the lower the quality and the smaller the file size. So to lower file size you can choose low quality and faster compression from the share out box. As you have mentioned that would reduce the quality.
Reducing the resolution could lower the file size slightly. You can do that by importing your shared out movie into QuickTimePlayer and exporting at a lower resolution.
Another thing to do could be sharing to email rather than file, that would give you a box with options to share at Small, Medium, or Large size, and at a lower resolution as well. Then drag the movie out of your email onto your desktop.
You could also re-encode the exported video to a lower bitrate by using a video encoder like the free download, Handbrake.
-- Rich
