In reply to:
... (the left one) is much larger than the other...
I don't agree. To me they look about the same size. Take in consideration that she is doing a TWISTED (not straight) situp/crunch to concentrate the workout to the sides of her abs and that would push her left breast higher and rest it in an angle on top of her torso and also lean against the right breast. The right one is sort of pulled down and slightly rolled to the side of her body by her twist. One of her folded arms is also hiding part of her right breast and the printed letter s on her top is off center that might confuse you.
So what made me think that it was a morph, or maybe a faked shot?
* The grain and fuzzyness could have been added after morphing to hide editing mistakes
* Low resolution, always makes it harder to see if it is morph or not.
* Strange shadows, that could be either from the combination of different light sources, or from editing.
* I didn't recognize her and since she was not showing her face, as if in consideration to someone who didn't want to have her face shown on the Internet, there are not many women on earth with the kind of proportions as in this image.
* The "easy" to edit monochrome top.
* How heavy it looks... he he
* The situation looked a bit too funny/amazing and relatively easy to fake for me to even consider it to be true.