Would you walk up to an overweight or obese friend of yours, grab their sides and comment on how you are worried about them and their health? Would you then say your comment was only out of love that you were bothering to publicly humiliate them? Your answer, like mine, should be a "hell no."
So why is it people feel they can do those things to someone who is skinny or underweight? What is the difference? I honestly would like to know.
I think it is all rude.