via https://www.xbmc4xbox.org.uk/forum/viewtopic.php?t=1571
ffmpeg -i some_1080p_infile.mkv -scodec copy -acodec copy -vcodec mpeg4 -b:v 3000k -maxrate 5000k -bufsize 4096k -s 1280x720 -f matroska -y outfile.mkv