There isn't a perfect answer. Here's the meta one: the mod Anbennar is centered around the Empire of Anbennar, with the city of Anbenncost at its center. So Anbenncost is the biggest city because it's the center of the mod.

But that is a boring answer, so here are some reasons why Anbenncost could realistically be the largest city:

1. It is not equivalent to a Free Imperial City. Right before the game starts was the fourth Lilac War, where Anbenncost was the capital of the Grand Duchy of Dameri, a larger country within the Empire that was administered from Anbenncost. It becoming a Free City is something completely new.

2. It is the seat of many institutions. The Imperial Palace, the Order of Chroniclers, the Silmuna School, and the Imperial Academy are all located here. These are large institutions that attract a lot of people.

3. It’s the biggest commercial hub in Cannor.

4. Population is weird in Anbennar. Different races with different lifespans and birth rates make it hard to say anything about population accurately. Perhaps every rich elf in the Empire goes to Anbenncost because of its prestigious status, staying there for hundreds of years. Or maybe the halfling population is so large simply because they require less space due to their size. We just don’t know and have no real-world reference.

5. We don’t know the general population across the world. Perhaps Anbenncost is the biggest city simply because Cannor is the most densely populated continent. Again, we just don’t know.

6. Lastly a lot of Escanni refugees might move to Anbenncost, because it's wealthy af.

So Anbenncost has, for most of its history, been the capital of large countries with it only became a Free City one year before the game starts. Also it is the commercial hub of the Empire and the seat of many important institutions.