How to decrease margin WITHOUT increasing overall page size?

Hi all,

In Micro$oft Word/Publisher does anyone know how to DECREASE the margin width's on the top, bottom, left and right of pages?

I tried drecreasing them all to 1cm width but then I found that it had actually increased the pages overall size! WTF!??!?!!

So anyway, I need a A4 sheet of paper but I want all that precious space the margine use up so how can I do this so that the page reamins at A4 size?

Thanks in advance.