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I was experimenting with JPEG quality loss upon resaving and noticed that not only (even after four resaves) did the quality not degrade (down to pixel level), but the file sizes actually increased by 200 kb or so each time.

Can anyone give an explanation for this? Where is this newly created information that increases the file size even coming from? I would have expected at least slight degradation and a decrease in file size. I saved using Photoshop’s save for web function and 100 % quality.

Original - 798kb

After 5 saves - 1020kb

*Original (top) - 798kb *After 5 saves (bottom) - 1020kb

  • I'm not sure but i think your image editing software adds some information into code of image. And this is increasing size. – Ferdi Çıldız Jun 5 '15 at 12:47
  • Does the file-size increase happen repeatedly? I.e., e.g., 1000 kb → 1200 kb → 1400 kb → …? – Wrzlprmft Jun 5 '15 at 13:34
  • Wrzlprmft, yes the file size increases repeatedly by approximately 200kb (give or take a few kilobytes) each time I save. – Joe Jun 5 '15 at 14:05
  • You don't see the difference in those two images? One is much more yellow overall. (repeated) JPG compression can often entail color reduction for colors with very similar values. – Scott Jun 6 '15 at 16:57
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Reason 1: Additional Meta Data

Use a tool like ImageOptim to get rid of all meta data and check the file size again.

If all files are (quite) the same size after removing meta data, you know where the extra bytes are.

A color profile might also be part of the file and therefore add some bytes.

Reason 2: Compression Artifacts

Saving an image with JPEG compression adds artifacts to the file. Even if they are not visible in the first place, the image data is different. Compressing an image with artifacts again, generates more artifacts and so on.

In this case, JPEG compression is not as effective as with the original file. The more artifacts are contained, the bigger the file will be.

Visualization of JPEG artifacts in your image

enter image description here

Technical details

If you are interested in the technical details, you might want to read this article:
JPEG compression algorithm

  • Okay I will give it a try! – Joe Jun 5 '15 at 14:07
  • I removed all the metadata and the filesize didn't change! – Joe Jun 5 '15 at 14:12
  • OK, interesting. Can you add two images to your question? The original file and one that has been saved several times. – Mario Jun 5 '15 at 14:16
  • I tested another image and I've just uploaded it. This one had file size increments of about 80kb. – Joe Jun 5 '15 at 14:33
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    @Joe: When the JPEG file is converted to RGB, most of the artifacts would represent fractional RGB values, but in most programs those get rounded to whole numbers. This rounding not makes the artifacts more visually severe, but it also adds half a bit of noise. The next time the picture is encoded, the encoder will try to compress this noise. Noise compresses very poorly, so the encoder will end up wasting lots of data to encode noise that was never wanted in the first place. – supercat Jun 5 '15 at 20:37

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