Why the hell does everyone think I am skinny?

I am 14 going on 15 in 3 months and I am getting really angry how people think I'm skinny. I am 5'8 which is 172 centimetres and I am 56 Kilograms which is 123 Pounds. Is it just that America has a different weight and height system or are New Zealand teens skinny? Or is really America just fat?