When you export with Best Qualtiy (pro res) you will get a .mov file about four times larger, or more, than if you export at just Good quality, that renders a Mp4 file. This is because of the much higher bitrate of the pro res format. So what you are doing is exporting at a much higher bitrate than the bitrate of the clip that you originally imported. Therefore much larger file size. Also, iMovie does not support 2.7k resolution, and that's why it got reduced to 1080p.
Additionally, when a movie clip is imported into iMovie, iMovie unpacks it for editing. If the clip were highly compressed it might have been a smaller size initially but will become larger when uncompressed for editing and then exported at with less compression.
Export at Good Quality 1080p instead of Best Quality (pro res) and you should get a movie with the same quality that you imported, with the exception that it won't be 2.7k. It will be a much smaller file size than than the pro res file.
--- Rich