I tried to point out to someone that they may be shooting a bit low for their height and got accused of being jealous, to put it politely. They were whining and complaining about being fat at just over 100 lbs at about 5'6" (exact stats blurred to avoid another nasty PM).

I tend to keep my opinions about excessive thinness to myself after that. It won't be seen as helpful, I guess and many of them seem to feel a heavy person is automatically wrong or jealous. What they don't know is I USED TO BE thin, and might have something to contribute... (disclaimer: not ALL thin people act this way.)