On a 2 minute clip:
ffmpeg.exe -pass 1 -passlogfile pass.log -i E002.avi -b 368k -maxrate 1000k -ab 62 -y out.webm ffmpeg.exe -pass 2 -passlogfile pass.log -i E002.avi -b 368k -maxrate 1000k -ab 62 -y out.webm
the bitrate still goes up to 2mbps. I tryed with -qscale 24 and it does the same thing.
edit
log: frame= 1323 fps= 20 q=0.0 size= 12348kB time=55.22 bitrate=1831.8kbits/s
-g 320 -qmax 63 seemed to do it!
'-g gop_size' Set the group of pictures size. default: 12
'-qmax q' maximum video quantizer scale (VBR)