Welcome to the community @Johanna_Lazar!
Are you talking about the difference between working in the Designer and previewing (clicking the eye icon) in the Designer? If so, that’s because your columns are taking up 50% of the screen width (with the image/content only taking up a portion of that) but enabling the preview expands the width of the visible screen—increasing the width of the screen.
There are lots of ways you could go about fixing the issue, however the easiest would be to limit the max-width on your columns element and centering it:
For reference, I only adjusted the top columns element in the screenshot below, showing that it’s contained to a narrower width than the bottom columns which stretch to the full-width:
Hopefully that helps, but feel free to reach out if you continue to have any issues/questions