It is hard-coded and there is no IC action that can change it.
Also, you can count yourself lucky. If your city could grow to 37k peasants, you would need to feed them, and the gains in gold are not necessarily commensurate. A low-pop/high gold city is ideal.
That seems like a missed opportunity though. Why wouldn't a thriving, productive happy city slowly grow in population? A city that can be fed should be able to grow, and a city that can't be fed should shrink, even if only 1 or 2% a year?
So much of Battlemaster is dynamic and responsive, that when I come across these artificially arbitrary/fixed limitations it's always a bit of a surprise.
