I'm guessing the window size is double, and not the actual icons. It's odd, I have a great graphics card, but whenever people do that on BYOND it all looks fuzzy as if it was improperly stretched.
It's actually because of an option set in BYOND. In the pager, go to "File" -> "Preferences..." and then go to the "Games" tab. If the "Use graphics hardware for displaying maps" button is not checked, check it. Otherwise, try it unchecked.