I suspect several factors are in play: primarily unreliable narrators. In this world, the size of armies and cities are often massively inflated. When reading the Anglo-Saxon Chronicles, for example, and the great battles between armies, it comes as a shock to understand that in the period, an ‘army’ could be thirty or fifty warriors, plus hangers-on.
Then there’s the factor that Westeros’ geology and climate seem to be subject to major magical cycles. The two continents seem to have shared a land bridge which has collapsed in historical times, and the destruction further east would have made the Thera eruption look puny. The mountain chains suggest some very violent uplifting, fairly recently, and doesn’t entirely fit plate tectonics. Then there’s the climate cycle, which seems to be driven by magic — not even the volcanology, and the thousands of years of recorded history. All these factors make applying terrestrial norms a mite tricky. Whilst Westeros may not be as magical as, say, Glorantha, it definitely isn’t Earth.