You cannot change the response of your sympathetic nervous system which is controlling your iris, which coincides with how large or small your pupil is at any given time.
As was said, the iris responds to a dark environment by relaxing, dilating (enlarging) the pupil to let in more light. Conversely in brighter environments the iris constricts making the pupil smaller for proper visual acuity.
There are a few medications that will constrict the pupil,.. ie: Alphagan an ophthalmic solution will constrict the pupil for a period of time (Rx only),.. as will most all opiate medicines (hydrocodone, codeine, etc..).
Again though, we can't willfully control our pupil size.