I've had stones that big as well. When I cut mine open it was actually made up of lots of smaller green stones inside. The whole thing was covered in yellow stuff (bile and/or olive oil perhaps?) I doubt it's possible for stones to be the size of a golfball without excruciating pain (followed by surgery to remove it). The small stones probably just get coated in bile or undigested olive oil as they sit in your bowel, forming what looks like a large stone. Try cutting yours open to find out!