ffmpeg — FFmpeg is an astonishingly powerful open source command line tool for converting, streaming and recording audio and video. (goodereader.com)
You will need to be comfortable using command line tools, and wiping your device for a fresh start with Android P. (cnet.com)
The only way I have found around this automatic re-sizing is to generate the Mobipocket file using the any2mobi command line tool provided with calibre. (kindleformatting.com)