Now it makes sense.
I guess it's more of a case of disliking Will as a character, not Billy Mumy himself. Billing-wise and age-wise, she should have been over him and not under him. (Marta Kristen was fourth yet did much less on the show, so...)
I believe Angela Cartwright earned less than any of the non-robotic cast members, but at least her character name is mentioned in the opening credits.