Stella is most probably following her big sister with this. Vanessa had a boob job a couple of years ago around the time of her modelling for Rhianna's lingerie collection but must've had them removed around the time of her pregnancy.
You have to stop comparing Stella to Vanessa, Even though they're sisters, they're two different people. Stella has remade her breasts because she works with her body. Stella doesn't do everything because her sister does it, huh