I think the name petite is referred to as someone who is small. Small does not mean short. They are totally two different things. You can be really small, tiny and not short. So when you see them call a lady of 180 cm petite, it could probably be because she has a small stature. My friend was actually 180 cm, but we called her petite because she not only looked small but was so tiny.
Petite is french for small. If for you a 170, or 180 is petite, than you might as well call an elephant petite.
Generally, people refer as petite to females that are small in height and/or their size.
You can call a 150, 45kg girl as petite.
You can call a 170, 55kg girl as petite.
You can call a 150, 60kg girl a petite, although personally I would not agree.
But calling a 170, 90kg girl petite is just ridiculous. There's nothing petite about that person.
And I'm not talking here to discourage metabolically challenged individuals.
If you want to pig out on 12 doughnuts for breakfast, do it, more power to you.
You're not petite, though.