Tvheadend

Source stream is 1080i@60 and that use 12,000Kbps bandwidth, IPTV stream.
I tried to transcoding with webtv-h264-aac-mastroka profile that use 384 vertical resolution(System log displayed 640*384).
However this transcoded stream still use 9,000~10,000Kbps bandwidth. In this case, I don't have any benefit to use transcoding.
How can I change the bitrate for this case? I want to set under 5,000Kbps for vertical 384.

Thanks,