AFAIK, the governments for the AI are placed by a script ( so, not building time at all ).
The speed to build siege equipment depends not only on the size of an army but also on traits and ancillaries of the FM leading that siege. Perhaps there is a hidden trait for the AI FMs, but i´ve never noticed that AI generals can do that quicker, though.
IIRC, this happens through adoption into another gens.
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