What you're seeing is the difference in screen widths. In the builder, the screen width (preview) is narrower than that of the live page because you have the Editor panel. Try collapsing the Editor and you will get a preview as close as the live page.
You can try playing with the Size & Position values. Those options are just the
background-position CSS properties. Please open this link for more details.
Because screen resolutions vary though, it's hard to set a perfect size and position that will look good for all screensizes from desktop to mobile. If you need to show everything in your image, it is best that you use the Image element instead.
You can see a setup difference between using a Section Background and using an Image Element in this screen recording.
Hope that helps.