I just compressed vp9 video from the game Syrian Warfare, from its original 1920x1080 to 640x360(and 320k bitrate). I got it from its original 160mb to... wait for it...
6mb. Here are the pics for comparison:
Original:
160mb.jpg
Recoded:
6mb.jpg
For those interested, ffmpeg string was simple(no filters etc.):
ffmpeg -i input -c:a copy -c:v libvpx-vp9 -b:v 320k -vf scale=640:-1 output
Thats it.