A size 0 has very little to do with body mass and more to do with bone structure.
Also there is a huge difference between a 0 and a 2 if you're a 2 or a 0 or 00 it matters a lot.
My GF is a 00 and eats more then me, but egged also 5'8" so finding pants means she always buys "big" with a 0 because they stopped making 00 clothing and also, kids clothing is too short for her legs.
A size 2 literally falls off her without unbuttoning...
She also eats about twice what I eat daily, I'm effectively a size 6 if I were a woman, and 5'8" as well.
Point is her bone structure is very small.
Some asian women for example are considered overweight and are size 0 but because of height or genetics are not the "healthy" size because the body mass of her genetic line is "normal" at a very thin (to us Americans) size.
Not all 0's are coke snorting anorexics... Just like not all overweight people are fast food eating slobs...
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