I’m posting more content from my old Pro Web Design Course (which you can no longer buy).
Are you confused by the difference between margin and padding in CSS?
And what effect do margin, padding, and border have on the width or height of an element (i.e. the box model, read more)?
This short (11-minute) video may help to answer your questions.
Note: The video was recorded 2011, so the browser versions are fairly old. But the principles still hold true.