She's 30 years old... How the f**k is this possible

She can do whatever she'd like but it's also ok for her to care about the opinion of her partner. She doesn't have to listen to what they say but there's nothing wrong with wanting to know what they'd think about it.

I agree 100% on that last part. The only thing I want to say is that where we store fat is genetic. The composition of each woman's breasts are different. A breast is made of fatty tissue, breast tissue, and lobules and ducts. A woman with denser breast tissue and less fat will not see as big a change in breast size with weight gain and loss, and a woman with little overall breast mass to start won't see as much as someone with more. In simpler terms, we can observe "shape types" in most women such as pear shape (more likely to accumulate fat around the lower body) or hourglass shaped. Diets, targetet exercise regimes, surgery, and wardrobe/support garments can effect a woman's appearance, but very few things will alter her predisposition towards a certain body composition. It's harder, or impossible to achieve some body goals for certain men or women based on our genetics. No woman needs permission to gain or lose weight though and should be supported in being happy with themselves.