Final JPG File Size 350% Larger Than Original

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2021-11-26 19:55:02

This isn't really a bug more a question about me wondering if I'm missing an option.

I just post-processed a few JPG files for a friend using Pixelmator Pro 2.3 (which is amazing) and noticed that the resulting JPG files were 300% to 700% larger than the originals. My post-processing steps were pretty light - mask out a bit of distracting stuff and then perform ML Enhancement colour adjust. I didn't perform any scaling but left the image at its original size when I exported it, at 100% quality, to a new file.

To get the file back to a size that wouldn't freak out my mail client (Mac Mail) I created a Quick Action to, no lie, scale the image to 100%. This reduced the file size back to about 200% of the original. I used JPG format throughout the workflow.

Here's an example of the change in file size during these three steps for one of the images:

Original: 3.0 MB
After post-processing: 21.6 MB
After scaling to 100%: 6.1 MB

Is this expected? I'm ok with doing a quick action to compress the files but thought I'd check to see if I'm missing something.
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2021-11-30 10:26:31

Hey Doug. Sorry for the late reply! Frankly speaking — yes, this behavior is normal and expected. At 100%, the JPEG quality is pretty much maxed out as Pixelmator Pro tries to apply as little optimization to it as possible (though some level of compression is still used even at 100% due to the lossy nature of JPEG format). That's why full-quality exports end up quite large and aren't that great for most web stuff. When the export quality is set to anything below 100%, however, that's where the compression algorithms really start to kick in. You can even go down to ~80% without much visible loss in quality but a substantial reduction in file size.

Since the Mail app probably isn't using 100% compression itself when transferring images, my suggestion would be to stay around 80% quality when exporting JPEGs — this should produce more web-friendly files right away and save you the additional scaling in the end.
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2021-11-30 21:00:53

Thanks for the awesome answer Aurelija! That explains things in a very understandable way.

Being a perfectionist but also one who doesn’t print on my own equipment the difference between 100% quality and 90% has been pretty undetectable to me. But ‘more is better’ so I’d default to 100% without really thinking it through. Your response gave me that chance to think a bit deeper and you’re right - the rest of my workflow doesn’t really need, or is able to accept 100% quality. Sort of like the realization I had after buying a 45 Mpixel camera. The resolution and sensitivity is great for cropping, astro shots and HDR but lacking an 8K monitor I have yet to see any of the thousands of the photos I’ve processed at 8K.