Her boob looks more like a ball under the shirt though :/, maybe try different position with it? Not trying to be rude please don't take it as that. Just thinking our boobs are not ball shape is all lol.
Naw it's not the position. Its the light, shadow and coloring that makes all the difference. See how her backside is in the light, with minimal shadow and uniform with the rest of her body, whereas the breast is in shadow, and harder to define. Proper definition, and detail for an object within shadow is HARD AS HELL to get right. It's a great image and you can tell the artist isn't a beginner with it. Don't judge him too harshly. n.n/