iMovie video conversion
From iMovie, go to Share-> Export Using QuickTime. Set the Export: menu is set to Movie to MPEG-4. Click the Options... button next to that menu.

Set the File Format to MP4
Set the Video Format to H.264
Set the Data Rate to something like 2048
Set the Image Size to 1080
Click OK
Click Save

That should give you a much better-quality video.

Hope this helps!