Why does a video file get bigger after editing it with iMovie?

Input File: - Kind: MPEG-4 File - Size: 305 MB - Dimensions: 640 x 360 - Codecs: H.264, AAC - Duration: 52:07 - Audio Channels: 2 After using iMovie to trim a bit from the beginning and end of the file, I tried to export the file via File → Share → File… with the following settings: - Format: Video and Audio - Resolution: 720p (iMovie only offers me 540p as alternative) - Quality: High (according to [an answer to a similar question](https://apple.stackexchange.com/a/208113) this means H.264) - Compress: Better Quality The estimation on file size shown by iMovie is 4.35 GB. My questions: 1. How come the file size *increases* after I cut something from it, encoded both with H.264? 2. Why does iMovie not offer me the input resolution as an output resolution? I'd like to be able to trim a video and save it with roughly the same size/time ratio.