Do you think the Dornish could, at one time, raise 50,000 men even though they can only raise 25,000 now? Could a pre-modern war – even ones as destructive as the various Dornish Wars – have that big and lasting an effect on a population? I always thought the population was more limited by geography – after all, Dorne is the same size as or smaller than most of the other kingdoms, and a lot of it is thinly-populated desert to boot.

Yes and yes, although there’s probably an element of exaggeration. 

On the premodern war part – if you look at the Gallic Wars, or Genghis Khan’s destruction of the

Khwarezmid Empire, premodern wars could involve casualties in the hundreds of thousands to millions and have a long-term impact on the area. 

And Julius Caesar and Genghis Khan didn’t have dragons raining down fire from the sky. If you go back to the WOIAF’s description of the First Dornish War, for example, pretty much every population center in Dorne was burned again and again and again. Add to that the total war practiced during Daeron’s Conquest, where not only did the Dornish (and the Westerosi) lose entire armies, but the Westerosi armies responded to guerilla warfare by engaging in reprisals on the civilian population. 

BUT! Key point here – a lot of this has to do with damage to infrastructure. One of the things about irrigation canals in otherwise desert regions is that they require constant maintenance and upkeep, otherwise they silt up and become useless. 

So I think it was a combination of intense warfare combined with an inability to maintain the infrastructure needed for agricultural production at the previous level.