TIFF Compression for bitonal images

Tiff images compress with color images, but the file size increases when I try to convert B/W images?? Can you suggest me where the problem might be?

           Stream imageStreamSource = new FileStream("3321103392-20-N-4201648.tif", FileMode.Open, FileAccess.Read, FileShare.Read);//3321103392-20-N-4201648
            TiffBitmapDecoder decoder = new TiffBitmapDecoder(imageStreamSource, BitmapCreateOptions.PreservePixelFormat, BitmapCacheOption.Default);
            BitmapSource bitmapSource = decoder.Frames[0];
 
            // Draw the Image
            Image myImage = new Image();
            myImage.Source = bitmapSource;
            myImage.Stretch = Stretch.None;
            myImage.Margin = new Thickness(20);

            System.Drawing.Bitmap rgbBitmap = BitonalConverter.Converter.ConvertToRGB(SourceToBitmap(bitmapSource));

            System.Drawing.Bitmap bitonalBitmap = BitonalConverter.Converter.ConvertToBitonal(rgbBitmap);


            FileStream stream = new FileStream("new.tif", FileMode.Create);
            TiffBitmapEncoder encoder = new TiffBitmapEncoder();
            encoder.Compression = TiffCompressOption.Zip;
            encoder.Frames.Add(BitmapFrame.Create(BitmapToSource(bitonalBitmap))); //imp//
            encoder.Save(stream);