The breast is the part of the body that the mother uses to feed her newborn infant.
Breast size and sexual attraction
Men vary in their preferences for breast size. A study found that men who are misogynists tend to prefer large breasts whereas white knights prefer smaller breasts. This makes sense because it is costly for women to have large breasts. Large breasts cause the woman back pain, and drain more of her nutrients because they produce more milk. A misogynistic alpha male would want his wife to have large breasts because he cares more about his offspring being well-fed than he does about his wife's health. A white knight would want his wife to have small breasts because he would rather his child be malnourished than his wife not be healthy.
Beta males are often white knights because they need a healthy wife in order to produce multiple children. Alpha males usually only reproduce with a woman once before moving onto another woman, so the long-term health of any particular woman is not very important.