What? - Fat?

Aitt, here is the thought for the day. How do you tell somebody that they are fat? Should you be polished about it and use real diplomatic words or just come out and say that? Things are easier when you are in a fight/heated argument. You can always yell stuff like "you are a fat pig" or "if you werent this fat, your brain could process stuff faster". What if the person who is fat is somebody you like and you werent in an argument/fight?