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 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:
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?