Thedas has the same problem as Westeros in Game of Thrones. The actual size of the world/continent is so small that it becomes implausible. As you can see in the maps below - the actual size of these worlds are not big enough to populate enough cities, farms, houses etc. for making a believable world. In both Thedas and Westeros, there are lots of groups, houses, great houses, farmlands, organizations etc - but in no way the actual land mass to support this as believable real life comparison locations.
Thedas is slightly better made than Westeros in this regard. Also this problem of "small scale" would also affect wildlife, nature and available resources - not only human population - making it even worse. In George R.R. Martin and David Gaider's defense, there comes a point where you have to consider practicality over believability and realism. If the maps where made alot larger - then what would suffer would be details. Like "Do I want this small nation to be very detailed, with every area being important and matter? Or do I want to create a much bigger area that uses "filler" land for realism?" (As I picture it, that is my opinion on the possible thought process).
Uh, Westeros is the size of South America if not bigger. It takes months to go from the Wall to Dorne if not on a ship. The North alone is said to be about the size of continental Europe. And Europe has incredible cultural diversity, so what you said just doesn't make sense to me.
As for Thedas, it does seem pretty small. It's mostly inconsistent, however; Origins implies that the Warden spent a year, if not more, traveling around the country. But an entire army can force march across half of it in two days, and an army is much slower than a party of less than a dozen. That would mean Ferelden is really small, like not even the size of Ireland small.
EDIT: also, doesn't Alistair say Weisshaupt is thousands of miles from Ferelden or something along those lines? Maybe it's hundreds. Because that gives us a decent base, it's basically on the opposite end of Thedas.