Well, there is no way for a camera to actually add 10 lbs (unless it's a 10 lb camera around your neck), so it has to be something about perception.
Front lighting is unflattering to me for a number of reasons. It's often sited as the reason one portrait looks like a "snapshot", while another of the same person, with side lighting looks more "professional".
An object's form is how we (as viewers) decide on that object's depth, volume, weight, etc... Form is created by combining shape with tone. Straight frontal lighting eliminates tone caused by falling light, and turns the object into a shape only.
Also it has to do with the subject's self-perception. My mother seems to think that if I never take a photo of her no will catch on that she is fat. To me, and other folks in my family, she looks pretty much the same in my photos as she does in real life. But to her, it's much more obvious that she is fat in my photos than in real life.
To see what the difference between frontal and side lighting is all you have to do it take a pic of someone with the standard in-camera flash most cameras have, and then take another with them standing near a window (to their side) with the flash turned of.
What to do about self perception isn't as simple. Good social skills and a little psychology might be necessary.