Allow me to quote the "Guide to the Senegal Parrot and Its Family" by Mattie Sue Athan (page 81).
A common and appropriate cage is 20 x 20 x 28 inches with a 3/4 space between the bars. A cage fewer than 18 inches deep can sometimes make a bird feel more exposed than protected because it cannot move back very far into the cage.
They didn't say that 20 x 20 is the minimum but rather a common size cage. Mind you that this was talking about a common cage size for most Poicephalus and not specifically for Senegal Parrots. Kili is on the smallest range of Senegal Parrots in size. And they only stated that a cage "fewer than 18 inches" would be too small. Basically the way I take this as well as from personal experience, 18x18 is the minimum size. I am well aware that my parrot is in a minimally sized cage. If she were in a below minimum sized cage and I became aware of this (as originally happened), I would immediately replace with something larger. However, as she is in a cage that is at minimum, I don't feel compelled to rush to replace it. Certainly when the time comes to replace the cage or if I come across some extra income, I will gladly replace with a larger cage.
From the routines we have established with Kili, she is not particularly active during the day (whether in cage or not) but is quite active in the evenings which is when she is normally out anyway. All I ask is that you grant me that she is within minimums for cage size. I full understand that bigger is better but replacing a cage is a fairly major expense that I would rather put off until necessary.
Also, I don't over stuff her cage with toys. Actually you'll probably disagree with me on that as well, but usually I don't put more than 3 toys in the cage. Rather I usually keep 3 toys in the cage. I do replace them more frequently but not too many at once. So I would say she has plenty of room to move around inside and lots of perches. But at the same time, she really enjoys being out and spending time together. The cage is more of a resting place than play area for her. She'd rather play together with me when she is out and relax when she is in cage.