there is stretchable space inserted before and after any multi-line display structure
amsmath is used. (stretchable space is similarly inserted around single-line
displays, but they aren't as susceptible to the surrounding context.)
take a look at what appears on the following page. if there is another large multi-line
display near the top (preceded by only one or two lines of text), it isn't by default
allowed to break the display if it doesn't fit on the previous page. instead, the
stretchable gaps (around displays, before section headings, between paragraphs -- all
to a different degree) are stretched to produce a bottom-aligned page.
this isn't really what you usually want, as you have observed.
if you are willing to allow the long display on the second page to break, you can approach
this in the following ways:
- if you are willing to allow breaks in displays throughout the whole document, place
\allowdisplaybreaks in the preamble;
- if you want to be selective about where a long display breaks, insert
immediately before the
\\ in the display where it is to take effect.
additional possibilities are described in the
amsmath documentation (
another possibility is that a box of some sort (e.g. a minipage) or other large
unbreakable element appears near the top of the "second" page. the tactics for
dealing with such a situation depend on what kind of element it is.