How windows calculate file dimension?

I Calculate the binary size of a file with this function:

public static int BinarySize(string path)
        {
            FileStream fs = new FileStream(path, FileMode.Open);
            int hexIn;
            string ret = "";
            for (int i = 0; (hexIn = fs.ReadByte()) != -1; i++)
            {
                ret += Convert.ToString(hexIn, 2);

            }
            fs.Close();
            return ret.Length;
        }

An example of my problem is when I calculate the dimension of this simple black PNG image (10x10 pixels)

With that function I find 640 bits => 80 bytes, but windows say that this file dimension is 136 byte. Why this difference of 56 bytes? Is the security, permissions or some private information that windows attach to every file?