graphicsimage editingpreview
An image saved locally on my MacBook Pro (macOS Mojave 10.14.3) has a size of 4.4 MB, as you can see in the screenshot below:
4.4 MB is the original size of the image, which has a resolution of 6000×4000. I want it to be 1000px wide. Opening macOS Preview to resize it, the software tells me that the size of the target image would be 152 KB:
Clicking OK and then checking the size of the new image, now macOS tells me it's 136 KB:
Why does macOS give me two different values?
The Preview does not lower the image quality, but will fit it in the given parameters.
If you use Preview to open a original image: Example:
5.5 MB file size with Dimensions of 3648 × 2736 pixels
Now open the Preview Tools - Adjust Size to see the current settings.
The default resolution is set to 72 which is standard for web applications.
In that window you can set it to the original size/resolution.
Now if you are using a copy/paste to move the image to Preview, the resulting quality will depend on the copy/paste settings. That can be found in the application it self.
Upon further investigation on color profiles after Agos's comment pointed me in the right direction, it turned out that using the incorrect color profile was responsible for the dither-like regions.
For others who have a similar issue, navigate to System Preferences > Displays > Color and select the profile that the image was exported in.
I would assume that the dithering-like region was due to the colors in that region being out of the color space of sRGB, and as a result the computer attempts to compensate.
Best Answer
Mac OS hasn't actually generated the new image when it gives the estimated size. With JPEG images, the content of the image affects the file size because image complexity impacts the ability for the algorithm to compress the file. What you see before completing the resize is Preview's best estimate.