Basics in Resizing Pictures or What happened to Picture Size?

I am writing a program that resizes pictures like this:

Image originalImage = Image.FromFile(pathToOriginalPicture);

Bitmap b = new Bitmap(newWidth, newHeight);
Graphics g = Graphics.FromImage(b);

g.DrawImage(originalImage, 0, 0, newWidth, newHeight);
g.Dispose();

b.Save(pathToOutputPicture, Ima开发者_运维技巧geFormat.Jpeg);

I tried to set:

newWidth = originalImage.Width;
newHeight = originalImage.Height;

The result was that the rezised picture file became ~900K while the original file was ~4M.

Why this is happening ? Is the quality of the original picture better than the resized one ? How?

I opened both pictures in Photoshop and I see that the original picture was 72ppi, while the resized one became 96ppi. Why is that ? Can I control this ?

Thanks a lot for your time !

You're not telling us the original format of your picture but you're saving as a JPEG:

b.Save(pathToOutputPicture, ImageFormat.Jpeg);

JPEG is a lossy compression format.

In addition to being lossy, JPEG also can output different quality (which is configurable).

This is what is happening to your file size: it is shrinking because you went, say, from a lossless format to the lossy JPEG or because you went from JPEG to JPEG-with-a-lower-quality.

Hence the size reduction.

Besides the format you need to set DPI, compression level settings etc. Check your Save function for overloads that will accept this type of input. See this documentation.