Its mostly genetics, so it won't be easy to obtain the "tennis ball" shape.
A couple suggestions:
1) Do more concentration exercises, i.e. Preacher Curls with an EZ Bar or Dumbbell Concentration Curls (using the inside of your thigh as a brace for your elbow).
2) Make sure you are squeezing and holding for 1-2 seconds at the top of each rep. Don't swing the weights or move your upper arms to assist your lifts.
3) Do cable curls. Vary handles and angles. Go slow during negative portion of rep.