It may just be the way you are built.
If you think there may be an imbalance (one side is stronger than the other and has thus gotten bigger), you might want to be sure to include one-legged hamstring work in your workout, being sure to work each side the same number of times (though I've also seen recommendations for doing extra reps or even an extra set on the weaker side until it catches up).
Always working hamstrings with two-legged moves like deadlifts, for example, can promote the imbalance by allowing the stronger side to do the lion's share of the work, letting the weaker side get by with less.
Moves like ham curls wearing an ankle weight, one-legged deadlifts, and stability ball raises with one leg on the ball (or on the top of a step or high step) are effective one-legged ham moves. "Legs and Glutes" contains several one-sided ham moves (one-legged deadlifts, some floor work with ankle weights)and PLB has some good stability ball ham work.