Un video (1920x1080) 156Mb -> (1280x720) 2,3Mb -> (640x360) 931Kb 

  VIDEO:  [H264]  1920x1080  24bpp  25.000 fps  165389.5 kbps (20189.1 kbyte/s)
  VIDEO:  [H264]  1280x720  24bpp  25.000 fps  2399.0 kbps (292.8 kbyte/s)
  VIDEO:  [H264]  640x360  24bpp  25.000 fps  964.1 kbps (117.7 kbyte/s)

Primero convertimos:
  # avconv -i video1.mov -vcodec libx264 -b 2500k -bt 50k -strict experimental -ab 56k -ac 2 -s 1280x720 video2.mp4
Esta opción comprime mas
  # avconv -i video1.mp4 -c:v libx264 -crf 28 -c:a copy video2.mp4
Luego redimensionamos:
  # avconv -i video2.mp4 -s 640x360 video3.mp4