You are using the built-in Rich Text Editor (RTE) to edit content on a SharePoint site. After you type a single line and hit enter, it adds an additional line space. If you copy multiple lines of text from another text editor like Notepad and paste, it looks good but when you hit enter in RTE, it adds an additional line space. This leads to an inconsistent user experience.
This is expected behavior at the time of creation of this article. By default, the Rich Text Editor formats paragraphs such as indenting and configuring the text alignment using in-line styles which might lead to inconsistent user experience.
When you type any text and hit enter in RTE, it adds the paragraph tag <p> … </p>. When you type any text and hit enter in Notepad, it only adds a <br/>' tag.
In the SharePoint RTE, use 'SHIFT+ENTER' to add a single line break.
SharePoint RTE behaves the same way that most HTML editors do. ENTER will add a new paragraph (with an additional space). When you hit enter in RTE, it adds the paragraph tag <p> … </p>…, which will show as a free line space.