Glaucoma Question, large cup size & normal eye pressure?

I went for an eye exam and the doctor said I had a large "cup" size, which caused him to recommend me for glaucoma testing, however, everything else was fine. He had mentioned that my eye pressure was normal, and that I may have been born with this "cup" size. How common is it for someone to have a larger than normal cup size and "not" have glaucoma?