Because of compression and different sizes.
The original JPG image you are referring to is 1080 x 790 pixels in size, for a total of 777k pixels. Since the image is true color, there would be 3 bytes per pixel in uncompressed form. That would require 2.3 MB. However, the image file is only 131 kB in size, which means it is compressed by about 18x.
JPG compression is quite good at deleting those parts of the raw image information that humans don't notice, or the human visual system is good at filling in. In this case the large amount of flat white background, and even more area that is out of focus helped a lot. Nonetheless, 18x compression is going to be lossy, meaning some information was deleted, not just clever use made of existing redundant information.
When the image is displayed on your screen, it has to be de-compressed. After all, each pixel on your screen used to display the image has to be set to something. The hand-waving of the lossy compression algorithm has to be interpreted to final concrete pixel values. Another problem is that the image may not be displayed on your screen at its native resolution of 1080 x 790.
Between these two effects, detail ends up being created that never existed. That makes it harder to re-compress the screen image. Most likely, no compression was applied at all to the screen image that ended up being 1.79 MB. The screen capture software probably doesn't apply compression unless you specifically tell it too. If you captured the screen image uncompressed, then 1.79 MB implies 597k pixels. That's roughly what you'd get from 1000 x 600 pixels, which is quite plausible for how this image may have been displayed on your screen.