You may not be able to achieve the same spacing on every page. Sometimes the factors involved just can't be juggled to make it work. A full-blown publishing typesetter system can "cheat" in ways that would be just too difficult or time-consuming in Word.

That said, the most common cause of big spaces at the ends of pages (with or without footnotes) is the Widow/Orphan Control setting in the Paragraph properties of various styles. Start by turning it off.

A full discussion of page length adjustment is at http://wordfaqs.mvps.org/BottomLine.htm, with special attention to footnotes at http://wordfaqs.mvps.org/BottomLine.htm#Footnotes.