That answer isn't feasible-- it's based on behaviors after the breakup, so they can't be the cause of the breakup, even if they were present (perhaps in less extreme form) before the breakup.

Disagree, it is most certainly feasible - and something I would consider a rather wise reason to break up with someone. Being in a position where you can do enormous amounts of permanent psychological damage to someone by telling them they have small breasts is not a good place to be.

Psychological vulnerability insecurity and a tendency toward self loathing are traits of a person (in the medium term) and are not impossible to predict. When you are breaking up with someone for this reason you are not obliged to wait until they actually spiral into self loathing so you can justify your decision.

The very decision to refrain from telling someone that you are breaking up with them because they have small breasts is based off their predicted response. So it is clearly just as possible to make the same prediction and have it influence your decision to break up with them because of their psychological fragility.