Caps for a single building

[woxihuanaa]

Freedom Tower(GS15.7x5 ), capacity 22750. It can't grow at all, with a demand bar 20000+ and region population 300w+.
After I changed the capacity into 21900 with Reader, only a tiny difference, it grows immediately.
The same thing happened to the Pan Pacific Building(GS15, 6X6, 22544) . After I decreased the capacity to 20900, it grows immediately
It confused me a lot.
Is there a cap for a single building? Or the cap for the capacity/area ratio?