If this is Word 2002 or 2003 and you have Track Changes enabled (and
balloons), Word displays a wide margin to accommodate the balloons. When the
document prints, it will be reduced to fit on the page.

Or it could be that it appears abnormally wide because the "white space"
above and below the margins is not displayed; see
http://home.earthlink.net/~wordfaqs/WhiteSpace.htm