: Younger breasts typically have more glandular tissue (which is firm). In your 40s, a process called involution begins, where glandular tissue is gradually replaced by fat. Because fat is softer, breasts often feel less "full" and more "squishy."
: Fluctuating estrogen levels can lead to water retention and breast pain (mastalgia) that may feel more intense or unpredictable than the PMS of your 30s.
: Estrogen dominance or weight gain associated with a slowing metabolism can lead to a noticeable increase in cup size. Conversely, some women experience a loss of volume as estrogen levels begin their ultimate decline. Health and Screenings