Simple question, I guess. The Heirogamicon says that research and production are modified by population. Is this the case?
It is...after a fashion.
With high population a city gains levels. Every level gets you a boost in money, research, and production, dependent on type. Conclaves get a lot of research, and then a little money and production for example.
But that said, the actual population number doesn;t factor in. Once a city gains a level, its actual population number doesn't hold any bearing.