What is the cause of an abnormal large clitoris?

Possibly normal. Great varuiation in normal clitoris size from tiny nubbin to about 1/2size of normal pinky finger. Markedly enlared clitoris is seen in in females with adrenogenital syndrome, in which case the clitoris can be confused with a penis and even the vagina is sealed. When severe is considered an ambiguous genitalia problem + needs a pedi endocrine dr + urologist. Androgenic tumors can cause enlargement.
See doctor. This could be a sign of too much testosterone in a female. See your doctor for evaluation (gynecology).