That's really the kind of thing you should be using page layout software for. In part, the behaviour may be due to the use of Word's widow/orphan settings.
It may also partly be due to paragraph before/after spacings and/or footnotes. You can modify Word's behaviour regarding paragraph before/after spacings by checking the options to suppress extra line spacing:
• at the bottom of a page;
• at the top of a page; and
• 'before' following a hard page break,
via File|Options|Advanced>Layout Options.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]