Sometimes, scaling down a bitmap generates a bigger file. Why?

Question

I'm trying to write a method to reduce the size of any image 50% each time is called but I've found a problem. Sometimes, I end up with a bigger filesize while the image is really just half of what it was. I'm taking care of DPI and PixelFormat. What else am I missing?

Thank you for your time.

public Bitmap ResizeBitmap(Bitmap origBitmap, int nWidth, int nHeight)
{
    Bitmap newBitmap = new Bitmap(nWidth, nHeight, origBitmap.PixelFormat);

    newBitmap.SetResolution(
       origBitmap.HorizontalResolution, 
       origBitmap.VerticalResolution);

    using (Graphics g = Graphics.FromImage((Image)newBitmap))
    {
        g.InterpolationMode = InterpolationMode.HighQualityBicubic;

        g.DrawImage(origBitmap, 0, 0, nWidth, nHeight);
    }

    return newBitmap;
}

Edit: Here's the missing code:

int width = (int)(bitmap.Width * 0.5f);
int height = (int)(bitmap.Height * 0.5f);
Bitmap resizedBitmap = ResizeBitmap(bitmap, width, height);
resizedBitmap.Save(newFilename);

Edit 2: Based on your comments, this is the solution I've found:

private void saveAsJPEG(string savingPath, Bitmap bitmap, long quality)
{
    EncoderParameter parameter = new EncoderParameter(Encoder.Compression, quality);
    ImageCodecInfo encoder = getEncoder(ImageFormat.Jpeg);
    if (encoder != null)
    {
        EncoderParameters encoderParams = new EncoderParameters(1);
        encoderParams.Param[0] = parameter;
        bitmap.Save(savingPath, encoder, encoderParams);
    }
}

private ImageCodecInfo getEncoder(ImageFormat format)
{

    ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();
    foreach (ImageCodecInfo codec in codecs)
        if (codec.FormatID == format.Guid)
            return codec;

    return null;
}

Answer 1

You are most probably saving the jpeg image with a low compression ratio (high quality).