Just by looking at the location, it's pretty ressourceful to have a good amount of food. Just look at the map. There's 2 rice ressources with a farm on it, one of them is next to a river, providing great amount of food. There's 2 wheat ressources, with a farm on it, next to a river, excellent for food. Add the farm adjency bonus when (+1 food for every 2 adjency farm in Feudalism) and yes, I am not surprised to see Krakow grow that fast.
So, correct me if I'm wrong... Both rice probably provides 5 food yields, both wheat probably provides 6 each, both farm between the wheat and rice probably provides 4 food and the North farm (near the rice) probably just provide 2 food. for a total of 32 of food just from those 7 tiles. Combine that with the river at Berlin's frontier, the one in the middle of the territory and the small river a the East. That place is filled with food yield.