Weight Gain
In some women, weight gains could alter the size of the breast. Breast tissue contains fat tissue along with lobules and ducts. So, when the fat content increases, the bust size could increase too; when you lose weight, the size could again decrease.
Pregnancy
It is quite natural to experience a feeling of heaviness in the size of the breast during pregnancy. Several hormones are responsible for pumping more blood into the tissues. In some women, soreness, tingling sensation and mild swelling could also occur. In some women, the growth continues till the final phase of pregnancy.
Contraceptives
Usage of birth control pills could also be one reason for the size variation. Those pills contain estrogen and could cause a slight increase in the bust size.
Puberty
During puberty, a surge of estrogen brings about lots of bodily changes. In some girls, the changes are gradual whereas in others, the swift changes could cause a bit of concern, but it is normal to experience increased size during that phase.
Intercourse
In some women, the size increases only during intercourse as the heart rate and blood pressure increase at the time of lovemaking. The veins may become evident and breasts could swell giving the impression of increased size.
Menstruation
After the day of ovulation, estrogen and progesterone make your breasts tender and fuller. This is because of a boost in the blood circulation to the breast area. Sometimes, water retention could also make them look bigger than normal before periods. If the size doesn't return to normal even after periods, then a gynecologist may help.
Menopause
After menopause, if the size has increased, it could be due to the fatty tissue. The proportion of glandular tissue may decrease and fatty tissue may increase which could make them look bigger than normal. Breast Lumps If you notice changes in the size and also feel the presence of lumps inside the tissue, you may need to visit a doctor once. In most of the cases, they are not serious but if those lumps are painful then it could indicate something serious.