Why are my nipples small?
For the same reasons that your hair color is what it is. For the same reason you are as tall as you are. And for the same reasons other women have large nipples: It's a blend of genetics and environmental influences.
Depending on your genetic make-up and your upbringing (nutrition, chemical exposures, etc.), you will develop to look a certain way. This may mean your nipples may seem small (they might not be -- have you been comparing with friends and normal people?). Hormonal influences during puberty make women's nipples and areolæ develop. During development, they expand and darken. However, this varies for folks. Some women will grow large nipples. Some will grow small nipples. Some nipples will be pink. Others will be brown. They'll even grow varying amount of fine hairs around the edges. All of these attributes exist in a gradient of normal variations. As long as nipples contain all the structures they need to function correctly, their size or color doesn't really matter.
For some more info, check out: