L'Anguille- Mostly arable, but with a rugged coastline and a stretch of the Forest of Arden at the bottom. No large towns, only notable settlement is L'Anguille itself.
Aquitaine- Largely arable, but with no large cities- even Aquitaine itself is noted to be more of a medium-size town. Large number of towns that have as many abandoned buildings as lived-in ones.
Artois- Almost entirely within the Forest of Arden.
Bastonne- Arable and pastoral, but with noted threats from the Forest of Chalons and the Black Chasm. Only notable settlement is Castle Bastonne.
Bordelaux- Good land, good coastline. Some trouble from the Forest of Chalons and from Mousillon.
Brionne- Good land, no real threats apart from disease, which is endemic.
Carcassonne- Little farmland, mostly pastures or mountains. Considerable threat from the Greenskins living in the Vaults.
Couronne- Farmland in the western part, mostly just horse-breeding in the Marches to the east. Rugged coastline. Considerable threat from the Greenskins living in the Grey Mountains and from Norscans over the coastline.
Gisoreux- Half of the population concentrated within the main hills area. Almost entirely within the Grey Mountains or the Forest of Arden. Considerable threats from both.
Lyonnesse- Rugged coastline, largely farmland. Main threats seem to be themselves.
Montfort- Almost entirely in the Grey Mountains.
Mousillon- Is Mousillon.
Parravon- Almost entirely within the Grey Mountains or a thin strip of Athel Loren.
Quenelles- Largely farmland and pasture. Mostly threatened by the Massif Orcal.
So, about half of the Dukedoms have considerable farmland. And even then, at best, they have a single notable city, no other large settlements.
I have no idea if Bretonnia has more people than the Empire, but I do know there's no source that outright says it. If people want to draw on factors present to say it does, that's their business, but that's headcanon, not canon.