I don't know it is a bug or not, but I play with sparta, and some cities cannot expand!
Their population is not growing, ant it always stays on 400 not less not more!
Heres some pics to understand what I meant:
Population growth -50% ??
It like the same all of the cities which have sign that the population is decreasing:
This effect is on: Messene, Phigaleia, Dyme, Sikyon, Kaphyai, Argos
When I conquered these city, there was no type of building for establishing control over conquered settlements, just a road, and walls...