It really depends on the strategy that the AI choses. This is a vast improvement over the Release-State of Civ, since there the AI seemed to know only a mixture of domination and science victory. Now some AIs might chose to built a smaller, but highly efficient empire, while other focus on rexing. From my experience their distribution is entrily based on the difficulty level you are playing. In my last Monarch game for example, I head three Civs, far away from me, who went for culture and each only had three cities. However my three closest Neighbors went for REXing and pumped out five cities each before the Year Zero. It might seem like a bad deal to you, but these small empires still can be a prick. They usually are top notch in science and can field a respecable defensive force, which, combined with their huge strenght cities cand make warfare against them really inefficient. And inefficient Warfare is what kills big empires.