It probably is because you're a little smaller - it could also be because of your activity level. Estrogen
is stored in your body fat, so if you have low levels of body fat, you probably have lower levels of estrogen, too. That's why so many female marathon runners, etc., stop having periods while they are in serious training mode.
I think if you have all the signs of it coming soon, it probably will sneak up on you one day and you'll wonder why you wanted it so badly! Stress
can also sometimes interfere with your hormones so if you are stressing about it, it could actually interfere with your body's normal time table/rythm, as well. So just relax and, if you want to, ease up on all the aerobic exercise
a little bit - your period will happen when it happens.
Hope this helps.