It's normal for women's boobs to fluctuate in size. Hormones, stress, and diet all play a role. She's always been thin (see photos below), which rules out weight loss.
Her boobs got smaller after she started actively modelling. It could be stress and adhering to a strict, imbalanced diet causing her boobs to deflate. As we all know, having big boobs puts you at a disadvantage when it comes to modelling. I can imagine Mia is trying to make her boobs smaller due to it.
A while back:
More recently: