A normal page setup for print documents is different than a normal page
setup for a web document. Depending on which version of Publisher you are
using, the default width of a web page is 760 pixels wide, where 8.5 inches
is 816 pixels, which is not a "normal" width for a web page. Web pages do
not have margins in the same sense as print documents.

I am guessing that what you are describing is the padding on the right of
the screen, which will vary with the width of the screen and the window.
Reference: Understanding background padding in a Publisher web (aka white
space):
http://msmvps.com/blogs/dbartosik/archive/2006/01/07/80563.aspx

If that does not answer your question, please post back.

DavidF