From the faq page http://www.publishermvps.com/Default.aspx?tabid=30

A.  There is nothing wrong with that. Publisher does code flush left which
is a Professional design standard. Your display has to do with your monitors
screen resolution and your browser window size in relation to the page width
you set your web publication to. In your web publication under File, Page
Setup, you set the desired width of the web page. Which is in relation to
the screen resolution. Increase the page width and you'll fill more browser
window space, or lower your screen resolution and you'll see less empty
space on the right.

DavidF