This sounds like it’s something you’d need to solve in your application’s CSS. I’d inspect the generated HTML and adjust the CSS accordingly. It sounds like it’s probably a margin or padding element you’d need to reduce.
https://docs.mendix.com/howto/front-end/customize-styling-new
Hope this helps
The CKEditor is deprecated and no longer supported. You could try a different widget. Thought Robert Price is right and you probably have to dive into the CSS