The file sizes are different because of how file compression works. Your image size is 3000 x 4000 which will not change unless you resize or crop your photo. Compression never changes pixel dimensions. It encodes the information. In jpgs case it encodes all the colors then decodes them when you view your file with any jpg viewer. Encoding photos (or mp3 or executable files) as opposed text files is problematic because there's generally not large blocks of information (colors) that are repeating (among other things).

Jpeg attempts to encode colors that are not exactly the same but very similar, so similar that hopefully only a computer can determine the difference (humans are limited in how many color variations they can see, computers are not) This works exceptionally well at moderate levels of compression. You can make it work less well with increased compression at the cost of file size.

If your picture has lots of color variation, it will not be able to group colors for encoding as well as it would for a photo with less color variation resulting in less compression and larger file size. Regardless, when it (your viewer) decodes (decompresses) your file, it will not only be the exact same size (pixel dimension) it was before compression, but will hopefully (99% of the time) look exactly as it did before compression.

It is important to recognize that jpg will not re-compress your file simply by opening and closing it. However, if you edit the file in any way after opening it, it will re-compress the file again if you save the changes over the original jpg. This will result in jpeg attempting to match similar colors again, and again, and again, each time you make changes. This eventually will start to look bad, just as if you increase compression levels in to begin with. Fortunately this is easily avoided.