If there’s a <p> tag somewhere in the CSS, see if there’s padding or margins. Every time you hit enter, you’re opening a new paragraph, so it must have something to do with how paragraphs are styled.
Two options:
Immediate: Instead of hitting enter, try shift+enter.
Long-term: Is there anything on your stylesheet talking about line-height? Get rid of that, or assign a custom line-height to the body or <p> tag.
