Fat isn't a nice word. Is hefty, hippo like, like a tank better because it is more factual?
If you have red hair and someone says you have red hair there is no offence.
If you are blonde and someone points out you have blonde hair, what would you think? That they were a bit of an idiot to point out the obvious?
Similarly, if you wear glasses and someone points that out you'd just think they were stating the obvious and being a bit unoriginal, may be trying to make conversation. Nothing else would be deduced from that.
If you are very heavy and someone said you look like a rhino would you think they were being rude or factual?
Whether a comment upsets someone or not depends whether the person is happy in themselves or not to take offense or not.
What is without doubt though is that the person saying it (and any person making any sort of comment or bad joke about how people look) is just a bit of a mindless idiot deficient in social skills.
Being deficient in social skills is a life long untreatable affliction.