Going to say what I said in the other thread
If prosperity increased food production it would cause a positive feedback loop of growth.
Prosperity has constant bonuses, the only thing that curtails it is food shortages, if massive towns/castles didn't have food shortage you could easily have a town with 100-200K prosperity. The food supply for towns/castles will also grow over time as the villages have a constant +1 to growth unless they get raided.
Now i'm sure there is a way to balance this so that you still have a food stockpile for your garrison but the idea that a massive city should just generate food simply because they're big is silly. Especially since in reality large cities can't survive for any time without supply lines from outlying communities. But as with all things, come up with a solution don't just complain.
I just modded the game files so that prosperity no longer lowers food stocks, This is 20 minutes of waiting.
That's a growth of 600 prosperity in less than 60 game days, there is literally nothing to stop the city from growing endlessly, technically my food surplus should be higher but I just put a base +100 to town food because that's the negative of 5,000 prosperity. Also militia growth is endless since its base growth is based on prosperity, so this would lead to towns and castles having hundreds of milita unless they are sieged down on a regular basis. Even a maxed out garrison of 450 would only provide a -23 to food stocks which is entirely offset and then some by the 2 villages and "surrounding lands" (+32) Also because the city is so prosperous, food intake from merchants is through the roof, Grain is 18 per unit in the city even with a stockpile of 350 (I think that rich cities generally have higher good prices since at this point nothing is average price in the city, even locally produced goods)