That depends. Are you talking length (from gum line to tip of the tooth) or overall size? If length of the tooth then they have probably been ground down due to tooth grinding (bruxism). In which case a night guard might help prevent further damage. If you are talking overall size then genetics.
Heredity/environment. Question if your teeth have always been small, or is it over time. The teeth can appear smaller if you are grinding your teeth at night, clenching during the day or both. If any doubts, consult a dentist.
Ancestors. We inherit tooth size from parents, grandparents, etc. Orthodontic treatment combined with esthetic restorations can do wonders. Talk to your dentist, and ask if results would be better if orthodontist created the best platform on which the dentist can do his restorative magic. Go for it.
Small teeth? How long have you felt your teeth were small? Are you a clencher or a grinder of your teeth as this can certainly contribute to the overall size appearance of your teeth.