Cookie nailed it,to elaborate a little further it's social heredity,what we're taught.e.g. this country has a strong sence of patriotism because we are taught about our forefathers that founded this country and there love for it,which is amazing seeing as how it's pretty much a prerequisite that you be Liberal to teach!
It's not okay, because some people may get annoyed with you if you tell them that they are too thin and some other people might take it okay! Same, most fat people are bothered if you tell them they are overweight and ogre would just take it easy! Good luck with that :)