The most likely reason is that you have the option turned on for the
Tracked Changes feature to use "balloons" for displaying the changes.
Word shrinks the main text to allow room for the balloons in the right
margin.

Go to Tools Options Track Changes and set the "Use balloons"
option to "Never".

If you aren't deliberately using Tracked Changes, you need to get rid
of them. First look at the status bar at the bottom of the window: if
the letters TRK are black instead of gray, double-click them to turn
off the tracking. Then click View Toolbars and display the Reviewing
toolbar if it isn't already visible. Click the down arrow next to the
Accept Change button, and select Accept All Changes in Document.

--
Regards,
Jay Freedman
Microsoft Word MVP FAQ: http://word.mvps.org
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On Sun, 30 Apr 2006 06:57:01 -0700, les.drewitt
wrote:

I have a multi page document that has a very wide margin on the right,
although the page setup says that the margin is one inch.