There is padding, or spacing, surrounding each text block, keep that in mind in case you are using a new text block for each paragraph. You could use one for longer, continuous text to avoid extra spacing. If text is copied and pasted into the editor, chances are that the code/format will also copy over as well. Check to see if there is problematic formatting that is adding more spacing.
If you are still having trouble with a specific campaign, and would like us to take a look at it, please feel free to email the campaign name and reference this post to: social_support(at)constantcontact(dot)com.
Since the option for adjusting line spacing isn't in product just yet, I'll track your interest. I also welcome you to share any product feedback or share what you'd like to see here in our board!