iMovie: how to export compressed videos

When importing, in iMovie 10.0.6, a video and then exporting (using the "Share" button) as a file, I realised that the new file is much bigger than the original (about 10 times bigger).

My question is how to export the file to original size (or equivalent) (i.e. using the same codecs I guess).

Thank you in advance for any suggestions.