I am not sure I agree with this. Founding more cities will speed up population growth. By the time a single city reaches a population of 10, two will be able to reach a combined pop of 20 (at least 18 or 19 if you count in the pop spent on a settler).
Growth is more limited by happiness than city sprawl, IMO. The only way those two cities grow so much is if they have food and happy and the happy isn't a given.
Small empires have some advantages, but they also have disadvantages.
- You'll likely control fewer resources and strategic resources, which will mean trade, city states, and maintaining relations will be more critical to keeping diversity of happiness providing resources and you could be drastically limited for strategic resources. A lot of that can be fickle if a large aggressor starts pushing around your friends. Larger empires are more like to control happy resources of their own, have larger quantities of strategic resources, and have a higher probability of later strategic resources appearing in their lands.
- If you're attacked, you're at greater risk of being annhilated or crippled - less to defend is often a good thing but if a stronger enemy comes for you and takes or cripples (via pillaging) even one city you could be in a world of hurt.
- Less opportunity to specialize or create military powerhouse cities even for defense. Getting all the +xp and military production buildings is a hefty investment. A sprawling empire could have several of these along with econ specialist cities, cultural cities, sci cities, etc. With a small empire pushing for a goal like science or culture you will most likely need to push for the goal with every city, and need to generalize each city, and maybe have one of the 3 able to crank out quality units. A large empire requires a lot more infrastructure but has the potential to have a lot more of everything and a bigger economy.
In Civ IV there was no real reason to keep your empire small. Even if you went for cultural you could sprawl and just focus on your 3 key cities. Mostly you were just looking to expand as much as possible in every game.
In Civ 5, the decision on what size empire to shoot for is strategic with consequences and something you might think about a lot before you even start. There could be good reasons to keep to a small empire, to sprawl, or to even go with just one city - beyond just doing it for variant purposes.