What causes large clitoris?

There are two...: Common causes! genetics and degree of arousal! not a bad thing, so do not worry!

Genetics, hormones: Or arousal. Some people are tall ; others short, same with size of clitoris. Arousal makes blood filled corpora (rods) erect ; thus large. Girls born with adrenal cortical hyperplasia can be born with a very large clitoris which can initially present as an ambiguous genitalia problem. Women who develop an androgen (testosterone-like) secreting tumor can experience clitoral enlargement.

Androgens: Any disorder with increased androgens could cause this. An ovarian or adrenal tumor, or even pcos could do this. However, it is also possible, and likely, that it is simply who you are and there is nothing wrong.