Your left breast is usually bigger than the right because of it’s close proximity to the heart
“Approximately two years after the onset of puberty (a girl’s first menstrual cycle), the hormone estrogen stimulates the development and growth of the glandular, fat, and suspensory tissues that compose the breast. This continues for approximately four years[clarification needed] until establishing the final shape of the breast (size, volume, density) when she is a woman of approximately 21 years of age. About 90% of women’s breasts are asymmetrical to some degree, either in size, volume, or relative position upon the chest. Asymmetry can be manifested in the size of the breast, the position of the nipple-areola complex (NAC), the angle of the breast, and the position of the inframammary fold, where the breast meets the chest.
For about 5% to 10% women, their boobs are severely different, with the left boob being larger in 62% of cases. This is due to the breast proximity to the heart and a greater number of arteries and veins, along with a protective layer of fat surrounding the heart located beneath it. Up to 25% of women experience notable breast asymmetry of at least one cup size difference.
If a woman is uncomfortable with her boobs’ asymmetry, she can minimize the difference with a corrective bra or use gel bra inserts. Alternatively, she can seek a surgical solution. Options include a minimally invasive procedure known as platelet injection fat transfer, which transfers fat cells from a woman’s thighs to her smaller boob. More invasive procedures include corrective mammoplasty, such as mastopexy, breast reduction plasty, or boob augmentation, depending on the nature of the asymmetry. Most surgeons will only perform an augmentation procedure to treat asymmetry if the woman’s boobs differ by at least one cup size”