The pupils dilate naturally in low light conditions.
They get bigger so they can let in as much light as possible.
WHen you are excited you engage the sypmathetic Nervous system,your fight or flght part of the PNS. This part of the nervous system that increases the heart rate turns off the digestive system sends...
Presumably in order to take in more light and thus gain more sensory input in order to react quickly to danger.
Not necessarily. Eyes dilate for many reasons.