What are some reasons one might have markedly unequal sized, asymmetric optic nerves and no symptoms?

The most common cause of asymmetric optic nerves with normal vision and no symptoms is a developmental or congenital condition called optic nerve hypoplasia. In this condition, the optic nerve in one eye is much smaller and underdeveloped compared to the normal optic nerve in the fellow eye. The other possibility is that one optic nerve is simply larger than the normal one in the other eye. Several conditions can cause this appearance, including megalopapilla and developmental abnormalities such as coloboma (when normal tissue in or around the eye is missing) or staphyloma (thinning of the outer whiter layer of the eye). It is important to know that any of these conditions can cause visual problems later in life. You should see an ophthalmologist who specializes in disorders of the optic nerve such as a neuro-ophthalmologist.