Why does macOS Preview show different sizes before/after resize?

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?

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.