But I'm comparing Poicephalus to Poicephalus. Their activity level may vary on an individual basis and on all honesty I don't know the complete difference between the species but I would suspect it be comparable. If I were to get Kili the size of the cage like for Cape, I'd have to build an entire room or aviary to match that for the Cape. I mean we can always go bigger and bigger to no ends so having some kind of size limit is practical. From a bigger cage is always a bigger one and you know we can't afford it in terms of price or the space it takes at a certain point.
I don't know if anyone knows but for the first week or so I had Kili in what later became Duke's cage. That green one you may have seen in videos. I had the cage initially for cockatiel, when I got Kili I told the size to employees and they said it would be fine. But since I had Kili in it for the first few days I saw her pacing a lot and seeming uncomfortable with it, not to mention barely fit through the door, so I went back and got the cage she is in right now. Obviously it was stupid to have her in the other cage to begin with but I was told it was ok.
As for cage size recommendations, it always looks like everyone is trying to step on each other to be proud to recommend a larger size as though it is an auction. Store employees try to upsell you and it is easy for strangers on forum to say to spend more money on a bigger cage. To measure how much you care about your parrot by the size of the cage (short of being absurdly small) is inappropriate because as I said there is always a bigger cage yet with the forest being the limit. No matter how many inches we add to the cage, it's still not like being out where there is much more room and things to do. In retrospect I would have probably preferred a few more inches each way but I don't see the upgrade worthwhile until the cage needs to be replaced.
The trouble is when inexperienced people are getting their first bird, they go by what they are told and if the store recommends a cage too small or too big, they end up stuck with it. There's no way of knowing the personality of the parrot early on and how that impacts cage size choice either. For that reason bigger is safer if possible but initially it seems like too much expense until you've had the parrot longer. Kili isn't too crazy in her cage so honestly I do think she has enough room. She spends a lot of time sitting in one place, napping, preening, or chewing. Even when she is out of cage and can go wherever she wants she's not that active. It's only in the training videos where she is doing so many things and moving around but normally she's much more mellow. I think with her personality it's enough cage size but if she were much more active I would have to more strongly consider an upgrade.