Fun fact, to achieve those kinds of washboard abs you see in supermodels, you need something like 4-2% fat content in your whole body! I think you also need to really dehydrate yourself to get that kinda chest*
*I could have no idea what I'm talking about and might be talking out of my ass so grain of salt.
Edit: Oh, and tons of exercise, too, obviously.
12% for women at a bodybuilding standard, 2-4% is the bare essential minimum for men so if a woman reached 2-4% they'd be having some SERIOUS issues, as their minimum is at 9-11% body fat. And keeping body fat at the minimum is likely to result in organ failure, so only contest body builders ever reach it, and only for a day or two at most (or else suffer the consequences). A fit woman has body fat in the range of 21-24% and works out. A ripped woman has body fat in the range of 14-20% and works out.
As for breast size, exercise helps. Posture also helps, when a man has his chin up and back straight it puffs out his chest and makes him look broader and bigger, works for women too. Mostly though, it's genes and hormones. Possibly epigenetic stuff too. Fat will be redistributed through the body accordingly to hormones (which can be affected by environment or controlled with modern medicines), but ultimately the hormones only work because of receptors which are controlled by genes. Some people are born for stuff, others are not.
There's also always surgery and whatever, but that aint real. Also dehydration is also just a contest body builder thing, as it makes the muscles more visible under the skin. Never heard it has anything in relation to breast size, if anything I'd suspect it'd decrease it.