In general, you need one food to sustain and two to grow... A city with one pop will alawys grow because the town allows it with 2 food...
There is a glitch in the game where it made my cities stagnent after losing pop... It just dropped to 1... There really need to be a "starvation" warning or something...
Too late now to find out exactly what happened, but like I said, when first captured, they're all unwilling to work, and I think they're just specialists once the city got back to size... Not to mention the borders were limited to where they can work too, so evidently some folks have to die from the lack of food...
Still, by time I noticed it, the pop is 1 and the food is stagnent... Makes one wonder how the city got started in the first place???