Ceasar's head doesn't fit at all between the bars and we spaced all of his perches and toys in a way that he can reach them, also, he loves to climb around on the bars of the cage so it's not a problem for him. The macaw cage is big but the spacing of the bars is safe. I would never put Kiwi in there though.
Ceasar's old cage is actually the same size as Kiwi's. I wasn't planning on changing cages with her, at least not for a while. The only difference between her cage and Ceasar's old cage is that the roof of hers has a slope and Ceasars is box shaped. If I moved her she would really only get a few more square inches of space in the top.