The size of the reflection on water doesnt correspond with the size of the solar disc.
I saw the same issue when using the Reflective Shader on a sphere object. The sun was too prominent at the default size so I cut it down to .00something and the size of the reflection did not change at all. 8)
I think the "solar disc" is a bit of a basic function right now, not fully integrated with the rest of the rendering system. This is why it doesn't reflect properly and also doesn't change the spread of rays from the sun. It may also be responsible for some issues with the sun disc showing through clouds. The look, behavior and controlability of the sun and other light sources will be improved in the future.