I'm: 34a, 125 lbs, 5'8"
purchased: small

i was going back and forth between xs and s. i decided on small because it fit better in my arms, didn't feel as tight. the xs fit well in the torso area and didn't have as much fabric as the small. the small also had a longer shirt tail. 

if you don't have as much mass in the arms, get the xs. if you have some muscle or extra mass in the arms, it's better to go with the small.