In short, bigger is better.
There is a common misconception circling the internet. Some people believe that if the bird has a large cage he/she will be content to stay inside, while a bird in a small cage will prefer being out and interacting with their owners because he/she will have a greater appreciation for space. This couldn’t be further from the truth. In fact, this logic is similar to saying that a human child would be better in a smaller bedroom, so that they want to spend time with their parents. Birds and children are quite similar, due to the fact that many parrot species have the same intellectual capabilities as a toddler, and have potential to respond to similar situations in the same ways.
Human beings enjoy spending time with their friends and will go out-of-the-way to make these interactions possible. Birds are no different. If your bird has a trusting relationship with you he/she will prefer to be with you than alone, regardless of the size of cage he/she occupies. By this logic, it makes sense to buy the largest cage you can afford so that the bird is happier during the time you cannot have him/her out interacting with you.
Additionally, exercise is very important for our avian companions. Birds will often grip tightly to a perch and flap their wings repeatedly (a behaviour known as “helicoptering”) in order to stretch and strengthen their muscles. If the cage is too small in any direction, the bird’s wingspan will be restricted and this behaviour will not be possible. A bird unable to exercise properly will develop stiff, weak muscles and arthritic joints. These health complications may shorten the bird’s lifespan, and cause them to become aggressive as a result of discomfort they are experiencing on a continual basis.
Finally, birds are similar to humans in that we both love our “stuff”. Human homes are filled with our possessions…figurines, books, electronics, tools…but as much as we love these items, we also appreciate organization. If our houses become uncomfortably cluttered, we discard unnecessary belongings and re-organize everything else. Birds are the same way in that they love their toys and perches, but need some unoccupied space where they can go to get away from the clutter. If the cage is too small, such space is not available and thus the bird becomes unhappy.