It's not about being "small" it's about anatomy.
Again, why would you play roulette with your life? Step right up, throw the dice, let's see if your pelvis has expanded enough to fit a baby!
During puberty, your body expands to become a baby making machine (sorry, it's the purpose of DNA). Your hips widen, your breasts become fully developed, and your body stores fat in a way that is more conducive to pregnancy. Girls change from being relatively androgynous at the age of 10 to a anatomical adult woman by the age of 16-17.
That is why 17 year olds suffer less bone-related problems during delivery. Their hips have already spread to a point that will accommodate passing a 7.5 pounds bowling ball. The stretchy ligaments that allow further hip spreading are also present (and I believe they are not present during puberty). Most of this is related to a rise in hormonal changes, which may, considering the dynamics of pregnancy, stop in a post-partum girl who hasn't completed puberty.
Take a look at some pregnancy related death statistics from Africa, where young (<15 years) pregnancy is more common. Such problems include prolapsed uterus, torn bladder, shattered hips, and a variety of painful obstetric fistulas.