Odd Phenomenon With Preview 11 on Mac OS 26.2

TidBITS Talk
1

I guess I will start with the question: What would cause Preview to delay accurate guaging of files sizes?

I am working with a HEIC file exporting and resizing etc.

Although things have settled down, the first iterations using the Preview Export function, and Tools > Resize function showed wild, wild display on file size.

The Finder > Size indicated a 6mb file, the various Previews functions indicated the file to be 11 mb.

After I exported, sliding the gauge to the least from 11 to 2.8 mb, when I further resized by opting to resize the jpg by 50% … it actually increased the size from 2.8 to 3.6 mb file size.

Things are back to “normal” … but there seems to have been a really strange process and delay of the system calibrating the file size… Unfortunately I can’t actually document or duplicate this since things are “back to normal”

Any thoughts

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Expect the unexpected when compressing an already compressed lossy image, especially using a different app, as different settings will be used.

It’s almost impossible to predict the size of a compressed image without actually compressing it, which takes time. I don’t know if Preview is doing that.

For predictable results start from a lossless source image if possible.

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I’m having some issues with file sizes and empty space estimates under 26.2.

Last night Pages refused to save a file despite having more than enough empty space on my internal SSD. I had to save it out to an external drive, reboot, then working normally

My internal drive seems to fill up overnight since the update and I’ve not changed any settings on iCloud, any apps whose data use significant space use external drives. But I’ve had to restart each morning. I’ll have to find time to get a bit more forensic on this.

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I just duplicated the issue I have, I won’t upload all the photos or anything, but the scenario is this.

Download a HEIC photo from iCloud 3.8mb >

Export HEIC-photo to jpg sliding file size option from 6.4mb down as low as I can to 1.6mb >

screenshot of finder info for the exported jpg is correct at 1.6mb >

Resizing jpg option 50% reduction actually increases the file size to 2mb.

While these numbers probably fall within parameters of the Finder accuracy,

the other day file size wild-anomalies were more in the realm of a factor of 10 times file size calibration

p.s. I think I even saw the file sizes change while I had Info panes open on the files, prompting my original query to include question regarding delay time of Finder “getting” a handle on file calibrations

5

As well as resizing this step re-encodes the file at different, probably average settings compared to your previously chosen lowest settings.

There are apps like JPEG Mini that would get it down again but before that I think it’s worth understanding what you are doing to these files.

Finder size info is accurate to the byte and instant. There is no uncertainty on the side of the computer.

Nothing about your process and the results is surprising to me. Happy to help shed some light if required.

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Just the first time I have ever seen jpg to jpg, simple, resizing to bump the size,

I will try similar on a system running Sequoia or earlier to experiment.

7

JPEG size is determined more by compression settings than by resolution. If you resize a heavily compressed JPG and save it with less compression, the file can absolutely get bigger even though the image dimensions are smaller.

Imagine two apps.

  • App A saves pictures by squashing them a lot.
  • App B saves pictures by squashing them a little.

Now:

You have a big picture that App A squashed really hard.

So the file is small.

Then you:

  • Make the picture smaller
  • Save it using App B

App B doesn’t squash it as much.

So now:

  • Smaller picture
  • But less squashed
    = Bigger file
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… but with a low quality image, because App B started with the output from App A, which almost certainly introduced lots of compression artifacts from the higher compression ratio.

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