High, will be a H.264 compression at approx 20Mbit/sec, this is a reasonable data rate for compressing HD video with good quality. ProRes, is a close-to-uncompressed algorithm that is optimised for editing video (computationally light to decompress and scrub through at the cost of lots of disk space). IMovie is optimised for working with video, not screen recordings, so the output options are tuned with that in mind. This efficiency is due to h.264 and that, unlike regular video footage, in screen recordings not much is typically changing between frames.

This is extremely efficient compression! (for comparison, standard definition DVDs rarely pack more than 4hrs of video onto 7GB of space). If you have ~5hrs of video in 4.73GB of data, that must be encoded at ~2.1Mbit per second. It is designed to store video using the least possible amount of data at the cost of using a lot of CPU time to encode or decode. H.264 is a codec optimised for distribution of video. The screen recording you made with Quicktime was initially saved using the extremely efficient H.264 compression algorithm. You have inadvertently wandered into the murky realm of video codecs.

How to export imovie to mp4