iMovie 4K Export Quality

I have some 4k videos that I shot that I'm trying to do some clipping and rotating on, but I recently discovered that when exporting, the quality actually decreases. To test, I shot a video, about 5 seconds long, in 4k at 30 fps. The file was about 30 MB. I popped it into iMovie, made no changes and saved it. It saved at about 16 MB. The dimensions of the video were the same, but the bitrate was almost halved! This was confirmed when looking at still frames I extracted and zoomed in on. Much more pixelated in the second one. I tried another video editor with identical results. Is there some video setting somewhere that is screwing up my video quality upon export?

I'm using the latest version on an iPhone 7.