Let’s cut to the chase: can losing weight affect the size of your boobs? If you’re currently on a fitness or weight loss journey the thought has probably crossed your mind. When it comes to the size and shape of our bust, genetics is mostly to thank (or curse, depending on how you feel about what you’ve got). Our genes govern the amount of fat cells we have, in turn determining the size of our breasts. As with the rest of our body, the fat cells in our breasts increase and decrease in size as we gain and lose weight – but it’s not that simple. The size of our breasts also depends on the ratio of dense glandular tissue to fatty tissue.