OpenCV + FFMPEG encoding bitrate

Hi to all. I'm in stuck since 3 days on the following problem. I have a small project in Visual Studio using OpenCV with FFMPEG and I'm encoding a video coming from a capture device. I can encode it with various codecs using the VideoWriter class without problem, including H263 and so on. My problem is that the resulting file have a bitrate of more than 1 Mbps that is not acceptable in my application. I need to reduce it but I couldn't find any way to set the bitrate of the encoder. Also googling for this results only in unanswered questions. Thanks