No one has literally sharp shoulders, but some look sharper than others.

This sense of sharpness comes from having less soft tissues around bone areas, so when you are overweight or exercise a lot your shoulders won't look as sharp.

I think there's also a tendency for Ds looking thinner than they realy are. I've been tracking my body fat because I had very high cholesterol last year. According to my doctor right now I have 30% body fat and should lose a few extra pounds (I think a normal % would be 25% or less), I've also been exercising 3x week because of that and have some muscles now, and still most of my friends are saying I look too thin and should get some weight. The body proportions create this illusion.