It's due to the wide angle nature of the phone camera lens. Angle plays a big part. Most phones default at .75 zoom, making that a wide angle perspective. Try backing up a little and then zoom in until you're at 1.00-1.50, and you'll notice the distortion going away. And like a previous poster alluded to, it also helps to squat down a bit to square yourself and the camera more evenly with the subject.