Ben, The pupil is nothing more than the opening created by a group of muscles in the eye. When light is shone into the eye, the pupil size becomes smaller and vice versa when light is taken away. That is the normal process.
Some people naturally have a large pupil.
If you are taking any type of medication or have allergies, this may be another reason that the pupils are larger. Forms of adrenalin in medication or the adrenalin released in your body's response to stress, may cause an enlarged pupil appearance. Too much caffeine, sugar, or any stimulant, are other causes, while a deficiency in B vitamins, particularly choline, is also a possibility. It's always a good idea to have your eyes checked for pathologies by a professional. Until then, keep your sunglasses handy.