It's frustrating to have a much bigger file exported from iMovie. For example, a editing of WebEx recording, which is MP4 file, in iMovie and exporting it out as MP4, will significantly increase the size, say from 300MB to 2.5GB.
The simplest way to shrink it, based on my experiments, is to use ffmpeg:
ffmpeg -i source.mp4 -c:v libx265 -preset fast -crf 28 -tag:v hvc1 -c:a eac3 -b:a 224k output.mp4