What avoids Bones from showing?

Let me just start by saying i am NOT skinny, im 18yrs 124Lbs 5'4 & a size 5 average, i don't understand why i'm "Big" you can say cause im not "small" but any how, i've lost ALOT of wieght i once weighed 172Lbs at age 12, very hard work & im still not happy... my body's not as firm as i would like... but the odd thing is my bones from my chest pop out & so do some bones from my ribs & my back? why? i am not nearly as skinny as lindsey lohan, or nicole richie(when their pics came out about anoerexia).. how come skinnier girls don't have this? i hate this.. how do i get rid of it.. or atleast get smaller to atleast have a reason for my bones to be popping out