It's common for portrait customers to complain that they look heavy in pictures. Of course, when they tell you this, they're really telling you that they expect you to make them look thinner, or at least as they see themselves. So, I've wondered, are they just viewing themselves through some sort of beauty filter, or is the camera really the villain?
Well, after mulling this over a bit, I've concluded that the added-weight thing is a real phenomenon. Now to some extent it can be due to posing, lighting, and angle of view, but primarily it results from a camera recording a two-dimensional, flattened version of our three-dimensional world. Our binocular sight allows us to sense depth and shape, even when the various surfaces of an object have similar levels of illumination. The camera can't do that, and that's why photographers must use shadows, highlights, and perspective to impart a sense of depth. Failing to do so effectively can leave your subject flat and fat.
To see where I'm going with this, consider the top view of a head shown in the neighboring line drawing. Notice how the distance marked as the "mask" of the face is narrower than that of the full face. Check out your face and the faces of those around you, and I think you'll find that most are definitely wider ear to ear than they are cheek to cheek. When we are looking at someone's face we look at the front surfaces that comprise the facial mask and unconsciously discard the sides. In the mind's eye, we end up with a narrower, smaller face than the one the camera sees. This is particularly noticeable when our subject is viewed and photographed in flat frontal lighting. Under these conditions, there is little shadow at the sides of the face and it is difficult to determine in a two-dimensional photograph where the cheeks end the sides of the head begin. A human will discern the mask; the camera won't. As a result, the photographed face will appear comparatively broad, spanning from ear to ear. As most photographs are taken in uncontrolled and often flat light, it's no surprise that people look heavy in many of their pictures.
The image to the left provides a good example, clearly showing how the facial mask (the highlighted area) is noticeably smaller than the full face. In this selectively-lighted rendition, our handsome subject appears as he does in real life. Imagine how this image would look if it had been photographed in soft, uniform lighting. The shadows on the sides of the head would be greatly diminished and the outline of the face would extend laterally from ear to ear. Needless to say, and I'm saying it anyway, our subject would look heavier and certainly not as he does in person.
Conclusion and Notes
Your camera is inherently biased toward adding weight. Use light and shadow to define facial contours, and, were possible, use hair or head coverings to define the edges of the face.
Using big light sources near the camera-to-subject axis or balanced sources placed on either side of the camera (copy lighting) is a sure way to make your sitter look heavier.