The rich text editor uses paragraphs when Enter key is pressed, and paragraphs usually have more vertical space around them.
In the signature, however, it's better to use line breaks instead.
There are several ways to add line breaks:
- Press Shift-Enter instead of Enter to make a line break, not end of paragraph
- Copy-paste from some editor, where line-breaks were used
- If you are familiar with HTML markup, edit the source code of the signature, replacing <p></p> tags with <br /> inside the signature markup