What is the cause of a large optic nerve?

This is a tricky question. A large optic nerve may be a completely normal variant. Nerves come in all sizes with some being larger than others while still being normal. However there are conditions such as optic nerve swelling that may make a nerve look larger. There are also congenital abnormalities and/or tumors that may make a nerve appear large. These things would not likely be missed in a full eye exam. I suspect that if you had a comprehensive evaluation by an ophthalmologist and were told that you have a large optic nerve then it is probably not a result of disease but is normal. However, if you are not sure then a full exam by an ophthalmologist should be considered.