How much more weight does a larger bone frame add?

I'm 5' 8", 140 pounds. But I also have a really large bone frame. I wear small shirts, size 3-4 jeans, A cup bra, and size 9 feet. But my frame makes me appear so much larger. For prom dresses my stomach fits a size 3-4 but I have to get a size 7-8 because it won't zip up my rib cage/shoulders (and I barely have boobs!) it drives me nuts because all of my friends are like 120 pounds and I'm recovering from an eating disorder, it feels so unfair. I feel fat, but my friends say it's just my large frame. How much more weight would that add?