>>423282564lol
Let me guess, you've been here all summer?
You've obviously never seen one of the many "chubby" or "curvy" threads filled with BBW whales that plague both /b/ and /s/.
It's not that the majority of the girls posted are "underweight". It's that most people (especially in America and Mexico) have a very skewed perspective on the subject.
Most of the women posted who are even midly curvy are actually overweight. The extent of which varies quite a bit. There is not one set boundry.
Conversely, there ARE naturally thin women who are at a healthy weight for their body type.
Try looking at xrays of fit human beings compared to slightly overweight ones. You'll find they are remarkably similar in skeletal proportion and the only difference is an excess of soft tissue on the larger person.
tl;dr most people on /b/ are fat apologists and probably need to go on diets themselves.