People say that pictures don't lie, but in one sense, they do.
Pictures are a two-dimensional representation of a three-dimensional form. Flash photography especially tends to flatten that three-dimensional form. When you then view the photo, you're perceiving a wider shape than there actually is. I'm sure you've heard that "the camera adds ten pounds," right? Well, if you're already carrying some extra weight, it adds more than that.
Here's a great article in
Slate that describes the phenomenon.
Another thing to keep in mind is that if YOU feel great and YOU know you look fantastic, then you absolutely do. I know that at my current weight for my height, I'm hardly model-thin--but I feel fantastic, so I suspect that I look lighter to others than my actual weight. An aura of confidence hides little flaws just as surely as a body-shaper or under-eye concealer.