Edible Parsley Roots

[JSmall]

I've recently seen in various food stores parsley roots for sale. They are about the size of small carrots. Now, I have grown parsley in my yard for a good while, letting the plants go to seed, but the roots are never as large as these in stores. Is there a certain variety that results in large roots or do the plants have to be grown in a certain way? The varieties I grow are curled and plain.