A client sent me a 420 KB photo, saying the file size was too low for a web form with a minimum requirement of 1 MB.

I opened it in Preview app, chose File > Export… and the JPEG quality slider was initially at roughly 80%. At that setting, the new file size was higher at 503 KB. If I dragged the slider to Best quality the file size increased to 1.5 MB.

Unless I'm incorrect in thinking this, the app can't add any new image data to increase the quality of the JPEG, right? I understand it might add some for a thumbnail but the photo already displays one of those. So what's Preview adding to the file that would increase its file size at all, let alone by over 300% for Best quality?


1 Answer 1


It could possibly be metadata. Try saving it for web.

  • I wondered that but the differences appear to be negligible. Plus a MB of metadata seems kinda crazy. Preview's Get Info results are nearly identical and a comparison of xmpmeta from Photoshop's File Info is the same, maybe 2 additional lines of text. I'm not concerned about reducing the file size, I'm curious to know why Preview makes such bloated files. However, I did Save for Web from Photoshop. At Max quality with a color profile embedded, the exported JPEG is 870 KB, twice the original. Its xmpmeta data is longer than the 2 previous images yet still smaller than Preview's export.
    – dugost
    Mar 17, 2016 at 21:07

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