While I agree that zoans usually are quite large, Kaku was fairly normal size for a giraffe being that they are huge anyway and Lucci was very big, but not in the same league as Boa's sisters - at least that is what I thought.
I mean looking at Impel's guard zoan's, they were pretty large, larger than the animal they have a form based on, but still not as massive as the sisters, especially considering that the snakes they are based, well cobra's at least, are not so great as to indicate such huge size.
Maybe it is just me, but there seems an appreciable size difference between the forms shown for Kaku, Lucci etc and the Boa's