The effect you experience is lens distortion. I think the most realistic "setting" is at 135mm or so. Obviously at 18mm there will be severe distortion. Its also affected by how far away you are from the camera and wether you are in the center or the edges of the picture. Its more severe at the edges. Also, the nearer you are to the camera, the narrower/longer your face will appear.
Here is some pics that make both effects more visible:
If you dont have a DSLR or other good camera where you can adjust that stuff, there are ways to correct lens distortion with photo shop.
Just google lens distortion and portrait photography.