I think there's something to be said for telling people hurtful truths that can be quickly remedied. If broached appropriately and with a right-minded heart, I think a comment about your haircut would have been taken just fine - because of the temporarily and ability to instantly or quickly be solved, it's not as offensive, and actually, it ends up being quite helpful. Like having body odor in one moment or a huge stain on our shirt.
However, if that solution takes time to be alleviated, such as being grossly overweight, ugly or simply stupid, these kinds of "honest truths" will cause true harm because they will cause strong self-esteem hits that will linger over time. I for one would be happy and want with these kind of "temporary hurtful truths" told to me. I would like to say I would be the same for the long-term ones, but I'm sure I would be truly hurt just the same.
Regarding 'grossly overweight' - every fat person knows s/he is fat. You're not telling them anything they don't know. And you're not going to give them an incentive to lose weight by doing so.
That's not even near true. Many people are completely oblivious about their weight. Though I highly doubt you will do anything but hurting them by telling them that they're fat.
Body dymorphia is a very real phenomenon, although it's commonly seen the other way around. As a formerly fat person, I have to say that I'd go long periods where I'd be willfully oblivious to my weight. The occasional hurtful comment and/or painfully bad photograph really did jar me into action.
Body image isn't always accurate. Surely you have heard of anorexia?
Clip from white chicks about that (opposite of a fat person thinking they are not, but similar principal I guess):
http://www.youtube.com/watch?v=6Gy4xMgArmk (dressing room scene -- possibly NSFW)
However, if that solution takes time to be alleviated, such as being grossly overweight, ugly or simply stupid, these kinds of "honest truths" will cause true harm because they will cause strong self-esteem hits that will linger over time. I for one would be happy and want with these kind of "temporary hurtful truths" told to me. I would like to say I would be the same for the long-term ones, but I'm sure I would be truly hurt just the same.