Meaning of “Do you have anything smaller?” from the dirty joke?

You’ve probably heard it before: A woman takes a taxi somewhere and she turns out to be out of cash, so she exposes herself to the cabbie and offers herself to him as payment, to which he replies, “Do you have anything smaller?”

What did that mean? That her body was so desirable that he’d consider sex with her to be overpayment? That her hooha was too large/wide for his liking? Something else?