Why are the margins in ArcMap so big or different than my printer's margins?
Answer
If Same as Printer (or Use Printer Paper Settings) is enabled in ArcMap Page Setup, the margins visible in the ArcMap layout are those designated by the Windows printer driver for the currently selected printer and paper size.
Many large format printers apply their required minimum margins to all page sizes; for example, large margins may be displayed on a small page size.