I don't understand how population work in this game. We only see category of bonus for more food you. Do we have the same population growth with 50 extra and 200 extra foods? Can someone explain me this plz.

Basically there's thresholds of surplus food, I think for an administered city you have a threshold at 10, 50, and 150 although I don't really remember (unadministered cities have higher thresholds.) So if you have 11 surplus in this example, you would grow the same as if you had 49 surplus food. So its a bit weird. The other thing that effects growth rate is how many pop are already in a city, so a 25 pop city that has reached the plentiful threshold will be getting pop faster than a 3 pop city at the same level of food surplus. Its sorta a neat system, but its really easy to hit max growth rate right now, and the different levels feel very far apart. To play optimally you have to micro all your farmers and its a real hassle. I think the system (and stability, could have a couple more tiers, so the levels aren't so crazy far apart.