[Question] How the heck Apple knows my body measurements?

Hello,

I bought Apple watch a few days ago. Today, when I was check Health app on my iPhone, I noticed that much more data has been collected since then, including my body measurements. And I must say, my weigth and height are measured really accurately!

But my question is how Apple found these pieces of information? I can imagine that wearing Apple watch enables measurents of heart rate, saturation, etc., but not body measurements. I am asking about particular algorithms used. The only ideas I have in my mind are related to some big data, and matching my behaviour and medical data to others, who have known weight and height.

Kind regards