The word "skinny" can have so many connotations. If someone is striving to lose weight and is told they look "skinny", then that is obviously a desireable thing. It's a goal. They don't necessarily mean the person looks too thin, just that they look smaller thlan before.
On the other hand, when people start calling slender people too skinny and talk about giving them a cheeseburger, I find that extremely offensive. I don't think that is any more acceptable than talking about how fat someone is.
It seems to be completely fine to discuss skinny people, critique them and call their arms "sticks" and say they need to eat more and gain some weight. Yet on the other hand, if someone is overweight, heaven forbid we call them on it and discuss it in a disparaging way. The double standard baffles me... mainly I guess because I am on the slender side. When someone calls an instructor's arms "toothpicks", I look in the mirror and see that my arms are the same size. Honestly, I don't care what anyone else thinks, but where's the compassion? Just WHY is it okay to say things like that about my (or other people's) body, yet not okay in reverse?
I guess it's an age old double standard that isn't going away anytime soon. My personal philosophy is, "if you can't say something NICE, don't say anything at all!"