AHAHAH!
Yes, I know exactly what's happening here. (What a nice change for me, to know what's going on!).
See attached images. In this image, you're seeing what you THINK you've got in your RTF:
https://www.dropbox.com/s/p4eixgn6am..._like.jpg?dl=0
BUT, if you open that file in Word and turn on the pilcrow to see the CODES in the file, you'll see what I see, which is this:
https://www.dropbox.com/s/eq6jzs6czf...raphs.jpg?dl=0
What you see in the 2nd image is that I've turned on the Pilcrow icon, which reveals non-printing codes; in this case, the PILCROW (the backward p), which indicates the end of a given paragraph--
every time it's used. Even "just" hitting the enter key creates a paragraph, albeit an empty one, if you use it to create vertical space.
So, your ePUB or MOBI or whatever views each line of the poem as its own paragraph, and thus,
the paragraph CSS rules apply.
As a WAG (Wild-a$$-guess), I'd hazard that you
probably have a top or bottom margin (or both)
for each paragraph. Thus, when your file is rendered, as each line=paragraph, you're getting the extra spacing. Right? Each line is its own paragraph--so the styling is kicking in, putting that top- or bottom-margin, or both, around each line.
That's my best theory, given what you've shared. If you want me or us to be certain, we have to see your CSS. You cannot "see" CSS, like CSS in HTML, in your RTF, per se. If you are using Word, or OO or LO, etc., you can see the information in STYLES, which is the word-processing equivalent of CSS. Styles and headings are Word's built-in superpowers. If you don't know how to use them, or what they are, where to find them, etc., speaking bluntly, you've horribly handicapped yourself, in terms of making/building an eBook, and equally frankly, you're handicapping yourself
every single day. Using Word without using Styles and Headings--you're using it like it's an electronic typewriter, not a word-processor.
Moreover, without a thoroughgoing knowledge of Styles, and how they work, I wouldn't hold my breath on being able to navigate CSS, either. For many of us, learning how to master Styles in Word was a big stepping stone to being able to easily handle CSS, when we moved up to working in HTML, eBooks, etc.
To see how to START to modify a style, in Word, please see this image:
https://www.dropbox.com/s/fy90ev3atx...tyles.jpg?dl=0 .
You would put your cursor in the line/paragraph you want to look at. You can then either a) click the style as it's shown, in the Styles pane, and click MODIFY, or you can b) open the style pane (see where it says STYLES, on the right?) and then click "modify style" from the resulting dropdown, or c) you can use one of the many other ways to get to, and modify, a style.
The margins will be found in the PARAGRAPH option in Format (see the lowest of the three arrows).
But...I didn't realize that you didn't have familiarity with any of this. I'd strongly, strongly recommend that you start here:
https://shaunakelly.com/topic/word/concepts.html
Work through her site, starting with items 1-->8 on that page, in the middle of the page. Then go to STYLES on the LEFT menu, scroll down the ensuing page to "For users" and work your way through those linked posts. Make SURE you read "Tips for Understanding Styles in Word." There are a bunch of other topic posts on this that will show up, on the LEFT, when you've opened the "Tips for Understanding.." also.
THEN, work through (back on the left menu) "Numbering, headings and outlines," which is the other half of the Word superpowers.
THEN, you will be ready to tackle modifying Styles in Word--and subsequently, modifying CSS in HTML eBook documents.
You might know some of this, but the fact that you didn't see what was happening, in the paragraphs, tells me that you probably don't. I'm also writing for others who may come along, later, and read this post. Shauna Kelly passed away some time ago, so sadly, her site will never be updated or fixed, but it's still a good place for beginners to get some of the concepts that are the REAL engine behind Word.
I hope that helps.
Hitch