c# - Reducing color depth in an image is not reducin the file size?

Question

I use this code to reduce the depth of an image:

public void ApplyDecreaseColourDepth(int offset)
{
    int A, R, G, B;

    Color pixelColor;

    for (int y = 0; y < bitmapImage.Height; y++)
    {
        for (int x = 0; x < bitmapImage.Width; x++)
        {
            pixelColor = bitmapImage.GetPixel(x, y);

            A = pixelColor.A;

            R = ((pixelColor.R + (offset / 2)) - ((pixelColor.R + (offset / 2)) % offset) - 1);

            if (R < 0)
            {
                R = 0;
            }

            G = ((pixelColor.G + (offset / 2)) - ((pixelColor.G + (offset / 2)) % offset) - 1);

            if (G < 0)
            {
                G = 0;
            }

            B = ((pixelColor.B + (offset / 2)) - ((pixelColor.B + (offset / 2)) % offset) - 1);

            if (B < 0)
            {
                B = 0;
            }

            bitmapImage.SetPixel(x, y, Color.FromArgb(A, R, G, B));
        }
    }
}

first question is: the offset that I give the function is not the depth, is that right?

the second is that when I try to save the image after I reduce the depth of its colors, I get the same size of the original Image. Isn't it logical that I should get a file with a less size, or I am wrong.

This is the code that I use to save the modified image:

private Bitmap bitmapImage;

public void SaveImage(string path)
{
    bitmapImage.Save(path);
} 

Answer 1

You are just setting the pixel values to a lower level.

For example, is if a pixel is represented by 3 channels with 16 bits per channel, you are reducing each pixel colour value to 8-bits per channel. This will never reduce the image size as the pixels allocated have already a fixed depth of 16 bits. Try saving the new values to a new image with maximum of 8-bit depth.
Surely you will have a reduced image in bytes but not the overall size that is, X,Y dimensions of the image will remain intact. What you are doing will reduce image quality.