为什么有些图像比其他图像大10倍

当我对Windows 7当前屏幕进行屏幕捕获时，生成的图像在300kb到3.5MB之间。是什么导致相同屏幕尺寸和相同质量的尺寸变化?如何才能避免这种情况?

这是我使用的简单代码:

int screenWidth = Screen.GetBounds(new System.Drawing.Point(0, 0)).Width;
           int screenHeight = Screen.GetBounds(new System.Drawing.Point(0, 0)).Height;
System.Drawing.Bitmap bmpScreenShot = new System.Drawing.Bitmap(screenWidth, screenHeight);               
System.Drawing.Graphics gfx = System.Drawing.Graphics.FromImage((System.Drawing.Image)bmpScreenShot);
gfx.CopyFromScreen(0, 0, 0, 0, new System.Drawing.Size(screenWidth, screenHeight));
bmpScreenShot.Save(@"C:''Snap.png", System.Drawing.Imaging.ImageFormat.Png);