How do I stop Safari Web pages from going to full screen

There are a couple of ways this could happen.

If you're running Stage Manager then it's designed to maximize the view of a single app, so turning that off might help.

Or maybe you've just maximized the window, so click the green button in the menu bar to toggle it, or drag one of the window corners to adjust the screen size.

You might have also toggled Full Screen mode via Command-Control-F (by default), so maybe toggling that will fix it.