Can I improve iMovie file export quality?

I’ve captured my old home videos as mp4 files using Elgato Video Capture, and the quality is quite good. But if I import a capture to iMovie, edit, and export as “High” quality, there is considerable degradation of video quality. Furthermore, the original file is less than half as large as the iMovie export version. I haven’t tried the next step up in quality, but iMovie estimates that the file size would be 32 GB, which is out of the question.

Is this what one should expect of iMovie, or am I missing something?