Ns should be grateful, or else they would stay fat forever.
Ironically, most of the SPs I know are fat. I pull an NP and walk to the shops to get something to eat, realise I'm not that hungry, then walk all the way back. That, or I forget to bring my wallet with me. Nness keeps me slim.
My mother who is an ESTJ is prone to this? She has the ability to keep mental snapshots of what people look like, so if she hasn't seen them for a length of time, she can notice if they've gained or lost any weight. If I gain any, she'll be the first to say, "Well your jeans are looking a little tight. Someone needs to take the dog out for longer walks!" My brother's long-term girlfriend has gained quite a few pounds, and my mum drops hints like, "Maybe you should join a gym?"
I would have thought that while STP types have good observational skills, their Fe would keep them from saying anything, out of politeness. STJ and SFP types have Fi; "sometimes things just have to be said." So, maybe ESFP types are the most prone, out of the SPs?
SJs are the kings of snapshot memory. An ISTJ friend of mine can remember the way people walk, talk, eat etc. Then he compares them to others and has the ability to quickly make comparisons of their physical traits to that of their behaviors, which are quite shockingly accurate.
Sherlock Holmes couldn't do it any better. But I guess he is an expert.
No way. I do take notice of physical appearance. But I never comment negatively on it. That would be just rude. But if I notice someone has lost weight or is in better shape, I might complement them (subtly) if the topic comes up.