Agreed that the ev-psych was bad. But...

If your true and actual reason for breaking up with someone is that her breasts are too small, consider that (a) saying "It was because you were too clingy" may cause them to try and mess with an aspect of their personality that doesn't even need fixing, and (b) total silence, which you may fondly imagine to be mercy, may result in her frantically imagining dozens of possible flaws all of which she tries desperately to correct, just on the off-chance it was that one. As opposed to, say, looking for a guy who's into smaller breasts next time.

Maybe I'm just being inordinately naive, but telling someone honestly, softly, and believably, your true reason for rejecting them, seems like it really should have certain advantages for them, if not for you. I mean, compared to either silence or lying. Calling it "grossly insensitive" is too quick a rejection of the possibility of telling a truth.