Are you using an LCD screen? If yes, it probably decided that since the application does not use the same resolution than the screen has, it puts the application in a window, so it doesn't have to calculate the pixels. An LCD screen is locked to a certain resolution and, if it has to display something smaller than that it either puts in virtual pixels to have simulate the higher resolution, or it puts everything in a small window, like it did to you.
My personal experience with LCD screens is that they are quite weird when it comes to what to do. Sometimes mine simulates the higher resolution and sometimes it puts things in a window. But it's quite stable; it rarely switches between the two states (with "rare" I mean every few months).
But I don't know how to set what it should do, sorry. Maybe somebody else does.
BTW, was my explanation understandable? I'm not very good at explaining things.