Double check that your game settings indeed are set to your monitor's native resolution for full screen. Just as an example, if your window is set to 1366 and goes to apply that setting to full screen it is going to stretch that 1366 to the monitor's resolution.

You can view the actual values being used by opening the confg file in notepad. Drawing a blank on the actual file/path and can't pull it up atm (at work, on phone), but think it was ffxiv.cfg in the root of the install folder. It is plain text and has two sets of values saved for windowed and full screen height/width. You can actually manually edit those numbers and save the file if you want.