The female metabolism is much slower than a male. Males eat more, drink more and still stay slim. And of course our metabolism slows down with age. This basically means we have to eat less and drink less. So watch your calories.
Females accumulate fat in different areas. We first accumulate fat in our thighs, butt and boobs. Because of our metabolism and very low T we need to exercise more to build muscle. We can have flat stomach but you have to focus and perservere. I do 700 stomach crunches daily for a flat stomach and for a solid butt I do 100 squats, I have a flat stomach,( except when I had a gassy stomach caused by a nasty bacteria from food I ate) and a solid butt from squats.
With diet and daily exercise regime you will find that your boobs sag and loose fat, sadly the only solution is accept your smaller boobs or have a BA.