You have to use a bigger curling iron or bigger rollers in your hair. Don't roll the hair so tight around the iron or roller so that you can get looser, bigger curls. Be sure to use hair spray to hold your curls.
Bigger curls require a bigger curler. Either find large curlers to put in your hair, or use a large-barreled curling iron. Also try braiding your hair after washing it and then drying it in the braid.