"Does this make me look fat? Well, does it? Be Honest!"

Being honest creates risk.

The risk of rejection, ridicule, dismissal,retaliation, anger, tears, etc. etc. etc.

Being "tactful" aka, being polite or "diplomatic" , allowing the other person to "save face" are all ways to make the other person feel better. Which, in turn, makes US feel better. We do it to soften the blow, not rock the boat, don't ruin the evening and don't upset anyone about anything , ever.

If being tactful is causing you distress, then you'd better find out why you are doing it. If you are seething inside, YOU are the cause, not the other person.