I’m underweight, so going with a much longer focal length is a no-brainer for me—but really, it seems like a bad idea to do portraiture with anything shorter than a 50mm lens. Less distortion aside, longer lenses let you shoot farther away from your subject, which generally makes them much more comfortable.
There’s a combination of factors that make you look fat. The focal length of the camera can flatten out a person’s features thus making them look much bigger in comparison. Barrel distortion—that is, when straight lines look like they’re curving out because of the nature of the lens—can also make a person’s face look more plump than usual.