I'm sorry I don't know how to create a minimal working example. I have a rather simple melody with a single staff (vocal). Under the staff I have two stanzas of lyrics (the first verse and the chorus). After the score, I have a \markup section with additional verses.
When I have just the first verse, everything looks fine. When I add the chorus, the spaces where I have \break become ridiculously large, as LilyPond tries to anticipate my needs, and forces the \markup section to a new page and vertically justifies / fills the page. On the surface, what I want to do seems deceptively simple. I've experimented with annotate-spacing and system-system-spacing to no avail. The truth is I don't really understand what I'm looking at with all the various distances. (In the annotated PDF, it looks like I want to shrink "extra dist (system-system-spacing)".) If I tightened the space between the \break's I think everything would remain on one page. Or, if I can't do that, and it has to force the \markup additional verses to a new page, it still seems weird to have so much whitespace between \break's. I think it would look better with all the whitespace at the end of the score...
