Two images with the same width and height , first one size is 500KB & second one size is 24MB , Why?

Question

I wrote two application first one : Generate image with random colors using Random Class in C# with range from 0 to 255 ARGB Colors for every pixel , the image size is 3000 x 3000 width and height . Second Application : Generate image with the same width and height (3000 x 3000) but using range from 60 to 120 for A , R , G , B of ARGB colors for every pixel ...

First App Generate image with size : 500 KB . Second App Generate Image With Size : 24 MB .

both of them uses PNG as format of the image and 32 bit color depth . i can't understand what is the difference between both of image , and why this differ in image size ?? and what things affect significantly the size of the image ? ........................................................................................................................... Sorry for my bad english .

this is the first app code :

public void GenerateImage2()
    {
        Bitmap Img = new Bitmap(3000, 3000, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
        LockBitmap LBM = new LockBitmap(Img);
        LBM.LockBits();
        for (int x = 0; x < 3000; x++)
        {
            for (int y = 0; y < 3000; y++)
            {
                Random Ran = new Random();
                Color C = Color.FromArgb(Ran.Next(0, 255), Ran.Next(0, 255), Ran.Next(0, 255), Ran.Next(0, 255));
                LBM.SetPixel(x, y, C);
            }
        }
        LBM.UnlockBits();
        Img.Save("redandrandom.png", System.Drawing.Imaging.ImageFormat.Png);
    }

second app code :

GC.Collect();
                int XDim = 0;
                int YDim = 0;
    int ImageDimentions = 3000;
int ForloopRange = ImageDimentions * ImageDimentions;
                Color CurrentColor = Color.Empty;
                Bitmap Btm = new Bitmap(ImageDimentions, ImageDimentions);
                LockBitmap Img = new LockBitmap(Btm);
                Img.LockBits();
                Random Rand = new Random();
                for (int i = 0; i < ForloopRange; i++)
                {
                    CurrentColor = Color.FromArgb(Rand.Next(0, 255), Rand.Next(0, 255), Rand.Next(0, 255), Rand.Next(0, 255));
                    Img.SetPixel(XDim, YDim, CurrentColor);
                    YDim += 1;
                    if (YDim == ImageDimentions)
                    {
                        XDim += 1;
                        YDim = 0;
                    }
                    if (XDim == ImageDimentions)
                    {
                        Img.UnlockBits();
                        Btm.Save(SavedFileName + ".png", ImageFormat.Png);
                        return;
                    }
                }

Answer 1

This is a (common) mistake as the instances of Ran are created so fast they don't get different seeds. Take it out of the loop to get real (pseudo-)random numbers!

    for (int x = 0; x < 3000; x++)
    {
        for (int y = 0; y < 3000; y++)
        {
            Random Ran = new Random();

Since many or most pixels will be the same in the first case, the image size is a lot smaller.

Answer 2

PNG is compressed format. One of them compressed better than another.