Sometimes telling people the truth isn't the right thing to do. I could go to a weight loss club and point out that all of them are fat, but that would just make people hate me. Just because it's true doesn't mean saying it is justified. Although I'm not saying you should lie about things either, especially if it actually matters.
Which is why I just said it goes far more in-depth than that. I can find an example of what you're talking about, where something simply doesn't need to be said.
http://www.youtube.com/watch?v=QjUJZVxtRT8There are always circumstancial variables. As Chrono said, this entire topic is much more in-depth and one couldn't possibly go and outline everything that is nice to do and everything that is mean to do - just like you can't with good and bad or right and wrong. Things change, circumstances call for different choices, and time can transform a decision.
We also have to remember that the world isn't black and white.
If you see said fat people, you don't tell them the truth by saying they're fat. They probably already know. There's no need to tell them. You also don't lie and tell them they're fit. This would make them over-confident, and will probably look for reasons to slack off. Silence would be the best option, and sometimes, people forget that they can just not say anything. (Not saying to be a mute, just be silent about that topic)
There is always more than two options.