The game seems to be designed for 16:10 aspect ratio, due to the globe in Geoscape looking round in 320x200. Right now the game is stretched to fill the whole window, which means it looks stretched or flattened if you're playing fullscreen with a 4:3 or 16:9 monitor.
It is true that most games that ran at 320x200 accounted for the fact that the resolution is usually scaled to 4:3, but as the globe looks oval when this is done, it seems XCOM wasn't designed with that in mind.
It would be nice if there was an option to letterbox the game to 16:10, especially since most video card drivers don't have 16:10 resolutions past 1280x800 enabled by default. This is merely so the game doesn't look flattened if you're playing it at 1920x1080, for example.
UFO Extender for the regular Windows version of XCOM has an option for this. It's an extremely trivial change, so I could probably add it myself if the devs have higher priorities.