X264 720 500mb

x264 is a free and open-source software library and a command-line utility developed by VideoLAN for encoding video streams into the H.264/MPEG-4 AVC video coding format. I tried to convert it to 720p with ffmpeg: ffmpeg -i MyMovie.mkv -r 60 -s hd720 MyMovie_720p.mkv. But I there is significant quality loss because bit rate decreased from 32.3 Mbps to 2.8 Mbps. How can I specify right bit rate for video stream, and left all other stream copied pristine?