I know size is often a big factor, and continous breeding is as well. I have a 4000 gram girl I bought this year that is an 11 egger in 2011, My spider girl was 3000 grams and she gave 9 in 2011, and 6 in 2013. But she went off feed really early in the 13 season. My 1500 gram het. axanthic girl gave 4. I think weight and size has more to do with it than genetics but I could be wrong.