I think you're assuming too rational a partner.

If you're honest and say, "Your breasts are too small," the person in question might seek a guy who likes smaller breasts next time. Or she might fall into a deep self-loathing in which she believes that her body is imperfect and nobody could be attracted to her, thus sabotaging her own future potential relationships. Or she could run out and get breast implants, even though she doesn't really want them, in hopes that you / other future guys will find her more attractive--which is much more expensive and possibly less rewarding than simply finding people who like small breasts.

In my view it's better to keep it vague. Guessing over dozens of possible flaws is likely to be less harmful than obsessing over one particular flaw, since it's difficult to figure out / change whatever possible flaw you think may exist.

(Disclosure: I have been dumped once and did the dumping once. The dumper kept it vague; I kept it specific but lied. I can't judge how keeping it specific while lying worked, since the person in question was bipolar and therefore not at all a normal test subject. I can judge how keeping it vague went: I obsessed over dozens of flaws for a while, until I found other people who were interested, at which point I decided it was probably just a bad match and nothing really to do with absolute flaws at all. I do not know how a completely honest dumping pans out.)

true reason for rejecting them

This usually makes little sense, particularly for someone one was attracted to for a while.

It's almost never true that for someone whose breasts one once found sufficient, her breasts would be a deal breaker, and no woman would be attractive with similar breasts regardless of her personality, face, legs, etc.

The problem is that the character sheet was filled out with mostly low die rolls, not that stat X is too low.

ETA: asking what the "true reason" for a breakup was is like asking what the "true reason" for a war, such as the Iraq War, was. Was it possible WMD? Past links to Al-Qaida? Possible future links to Al-Qaida? Past human rights abuses such as mass torture and murder? Aquiring influence over oil? Creating a pro-western regime? Creating a democratic regime? Perceived divine guidance during Bush's praying?

The first test to figure out if someone is more rationalist than emotional about the Iraq war to ask them what the "true reason" for the invasion was and see if they right that wrong question. It's just as much the wrong question in this context as that one.

Calling it "grossly insensitive" is too quick a rejection of the possibility of telling a truth.

I agree.