In addition to duration of the movie, file size is determined by how high the bitrate, the frame rate, the original size of the file, the format used for export, the compression, the resolution, the complexity of the movie and its edits. You can vary any of the above to reduce file size, (lower the bitrate, original file size, frame rate, complexity, and/or increase the compression -- although I would not recommend fooling with the frame rate.
If you export at Best Quality (pro res) for example you will get a .mov file that is 4x file size than an Mp4 export due to the much higher bitrate with pro res and much less compression. Therefore if you export the same movie at any setting other than Best Quality (pro res) iMovie will render a an Mp4 file of much smaller file size.
In your case the original video clip may have been highly compressed. When iMovie unpacks it for editing it may then export it at iMovie's compression rate, that may have been lower than your original clip, thus larger file size. The bitrate might also have been higher than that of your original clip, again increasing file size.
-- Rich
