it doesn't help them. you can be honest without being critical. if the dress looks great on her even though she's a bit chubby, a "no" is ok. but if you notice it about bust at teh seams, it's time to give her a honest assessment. i'll give you a personal example. i took up photography a couple of years ago. started posting pics on FB. all my friends seemed to like every photo i posted even tho i knew they were crappy by photography standards. although i appreciated their kind words i didn't grow as a photographer until i posted my photos on a photogrpahy site for critique. they were honest with what i needed to work on. and that is what really helped me improve.
Shayan, I believe there are ways to say the truth without insulting or offending. I think they get mad because they don't want to hear that it's through a fault of their own that the situation or whatever the "truth" is that. They want to somehow leave the blame somewhere else. Now, what did you say to that girl, and can we get pictures? The answer is ALWAYS NO because she's already fat, she didn't need ____________ to look fat. She was fat to begin with. Mrs. SwoLy has NEVER EVER asked me that question. She knows I WANT HER fatter. I likes me some BMW.