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I took a stock JPG photo and cropped it to 3840x960px. Then I loaded it into two apps: Gimp and Photoshop.

I used Gimp 2.10.6 File > Export As... dialog, changed the quality to 85% and clicked Export. The resulting file is 407KB

enter image description here

I used Photoshop 2020 File > Export As... dialog, changed the quality to the same 85% and clicked Export. The resulting file is 1.2MB wich is roughly 2x larger. Didn't expect that :)

enter image description here

Why is the JPG file exported by Photoshop roughly 2x bigger? What am I missing here?

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  • Well, I guess export algorithms are different? Btw I’d try Photoshop Export As instead of Save for Web: it should use more modern JPG exporter
    – Sergey Kritskiy
    Commented Aug 28, 2020 at 16:02
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    @SergeyKritskiy - The OP said they used Export As. That may be the problem. It doesn't allow control over all the JPEG saving options. Using "Save as" gives more options. Although I'm not entirely sure why anyone would expect two completely different applications to have the same results anyway.
    – Billy Kerr
    Commented Aug 28, 2020 at 16:08
  • @BillyKerr my assumption is the JPG quality percentage is defined in the JPG standard so it should be handled in a similar fashion across apps
    – hidarikani
    Commented Aug 28, 2020 at 16:15
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    Also note that it's not a good idea to recompress JPEG images in any application, since it will degrade the quality. The best results are obtained by compressing an original uncompressed file, such as a RAW, or TIFF, etc.
    – Billy Kerr
    Commented Aug 28, 2020 at 16:29
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    Have a look at the JPEG FAQ - while dated, it provides insight into how (not) comparable settings in different applications are: faqs.org/faqs/jpeg-faq/part1/section-5.html - GIMP follows the IJG scale (i.e. what libjpeg uses), and this isn't a percentage.
    – Michael Schumacher
    Commented Aug 29, 2020 at 21:08

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The assumption, that image processing apps use the same quality scale is invalid. The quality scales are not standartized across programs. A more detailed explanation can be found in the JPEG FAQ as suggested in the comments by Michael Schumacher

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  • I think its more likely that the bulk of the difference is that photoshop embedded more metadata
    – joojaa
    Commented Aug 31, 2020 at 12:34
  • @joojaa that's a good point, I'll test that...
    – hidarikani
    Commented Sep 3, 2020 at 13:27

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