When you use the built-in Rich Text Editor (RTE) to edit content on a SharePoint site, an additional line space is added when pressing the ENTER key. This issue does not occur when you copy and paste multiple lines from a text editor.
This is expected behavior. By default, the RTE formats paragraph indents and text alignment by using in-line styles. The styles are not consistent among text editors.
When you type any text and press ENTER in RTE, it adds the paragraph tag <p> … </p>.
When you type any text and press ENTER in Notepad, it adds a <br/> tag.
In the SharePoint RTE, use 'SHIFT+ENTER' to add a single line break.
SharePoint RTE behaves the same way that many HTML editors do. The ENTER key will add a new paragraph (with an additional space).
When you press ENTER in RTE, it adds the paragraph tag <p> … </p>…, which will show as a free line space.