First thing that might be helpful to know is that every time you hit the Enter button when you're editing, Drupal will start a new paragraph (as opposed to simply a new line). That's the reason for the extra spacing — i.e., it is adding a space between paragraphs.
To remove the spacing:
1. Go to the page you want to edit. Click New Draft.
2. Go to the Primary Body section and click the Source button.
3. Notice that Drupal currently has paragraph tags (<p> and </p>) between lines.
4. Delete the </p> tag from the end of the first line. Replace it with a <br> tag. Then go to the beginning of the next line and delete the <p> so that Drupal does not start a new paragraph. Example below:
5. Save the page.