Many women regard their breasts as important to their sexual attractiveness, as a sign of femininity that is important to their sense of self. A woman with smaller breasts may regard her breasts as less attractive.
Source:
Breast Size Perception and Satisfaction, Body Image, and Psychological Functioning in Caucasian and Asian American College Women, by
Elissa Koff & Amy Benavage, https://link.springer.com/article/10.1023/A:1018802928210