I think it is always extremely rude to call someone fat, doesn't matter what your own size is. But yeah, some people just make you want to ask them if they own any mirrors. Not very long ago I found out about one of my boyfriend's mates: apparently he keeps telling him I am fat. Which is kind of obvious, not really a secret, is it? It's not like he's opening my bf's eyes to a universal, unknown truth. Needless to say, I got pissed, but I found some kind of twisted comfort in the fact that this specific gentleman is ugly as hell and has to look way up to look me in the eye (and believe me, I am quite short).
Nevertheless, I'd much rather have him keeping his opinion to himself. Doesn't it make you angry when people have the urge to judge your appearance out loud? Even if their opinion is generally positive, I really don't appreciate it. If I want it, I'll ask for it, thanks.