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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:

enter image description here

4.4 MB is the original size of the image, which has a resolution of 6000x4000. 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:

enter image description here

Clicking OK and then checking the size of the new image, now macOS tells me it's 136 KB:

enter image description here

Why does macOS give me two different values?

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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.

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    Yup. In short... it's a guess. ;) – Tetsujin Aug 2 at 17:50
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In addition tp @samh correct answer, when you save a jpeg in preview you get a slider to set the quality. Underneath the slider is the file size. You can see how the size varies with quality. I don't know what determines the quality but it effects the file size.

  • By reducing the quality, you are reducing the amount of instructions being saved to recreate the image. Fewer instructions to the computer means less stuff store in the file. This means the file will be smaller, and the result will be sloppier. – samh Aug 2 at 19:34

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