Hell might as well spend an extra hour photoshopping a fat model's whole body instead of just her face since they are cheaper than thin ones. In 2016 I don't even know why models are even needed, CGI is good enough to be convincing completely from scratch with "models" that don't really exist.
As a model myself I can guarantee you that 100% of the photos that are taken for professional outlets are being photoshopped. Some more, some less but there's always at least a change in lighting to enhance the skin tone/color of the clothes/general mood of the photo etc. They are usually not photoshopped so the model looks like a completely different person but yeah there's definitely some "enhancement" going on.
In this particular pic it may seem photoshopped but again there are possibilities that she is naturally so plus it's a still pic hence the lighting, camera angle, make up etc will make a difference from frame to frame
All models are photoshopped a bit. But what you have to remember about plus sized models is that even though they're plus sized they're picked because they're the most beautiful plus sized girls, which is why naturally they will look better then most plus sized girls.
I've noticed that too with some of them, and I think it is mostly the makeup they are wearing. Contouring can make your face appear slimmer. The photos are usually edited to some degree too, so that may be part of it.