Even the babies of New Zealands are typically only about 3-4 inches long at birth (maybe half again the size of a Netherland Dwarf baby). As I recall, your little guy was the only kit in his litter. When a doe conceives only one or two kits in a litter, they tend to go a few days past the normal gestation, and grow a good bit bigger. Typically, birthing these babies is such a laborious process, they don't survive. He was an unusually large kit because of his "singleton" status, and he's a very lucky little rabbit to be alive!