I can understand it as a description from a young child but not one over the age of say 8. I have 3 kids 4,6,and 8 and I do think it is bad manners for them to call someone fat or refer to anyone by the way their body looks in any way unless they are describing them and at 6 & 8 both of my daughter's know the difference between name calling and description.Too many parents don't teach their children to be respectful of others and I confirm it every time someone compliments my kids for what I consider normal behavior .
I have been called fat by young brutally honest kids (hurtful but not as much knowing their intent) and also by poorly raised teenagers (that pissed me off ) My mom is thin and she never wanted to be called skinny so I think that can be equally offensive. Name calling is poor form period.