You’ve complained about the world of ice and fire saying that the field of fire army of the reach and west was too small to be the largest army of westeros and seem to treat canon times kingdoms’ army and population size as being the same going back centuries or more, but doesnt that ignore population growth? It could well be the case that the field of fire army was the largest because the population of westeros was much lower in the past.

Premodern population growth is extremely tricky to generalize about, because you have some rather significant changes from period to period:

• Late Antiquity (200-600 CE) saw significant (almost 30%) declines in population, due to repeated plagues, the collapse of the Western Roman Empire and attendant wars and decline of trade, and a decrease in global temperatures that reduced crop yields. 
• Early Middle Ages (600-1000 CE) was pretty stagnant in terms of population growth. More plagues, dislocation due to various invasions, low levels of productivity, etc. 
• High Middle Ages (1000-1250 CE) saw a huge population boom. Huge areas of wilderness were cleared and cultivated, the climate got warmer which meant that growing seasons got longer, relative political stability made new settlements less risky, trade expanded and with it so did urbanization, etc.  
• Late Middle Ages (1250-1470) saw significant declines in population during the crises of the 14th century, including both the Great Famine of 1315-1317 and the Black Death (which all on its lonesome wiped out about 50% of the population of Europe). 

But beyond that, we don’t see evidence of significant Westerosi population growth in recent years. King’s Landing isn’t noticeably bigger in population in 300 AC than it was in 26 AC when the walls were completed; Oldtown’s population doesn’t seem to have changed either in that period. The North’s armies stood at 30,000 strong at the time of the Conquest; they’re not noticeably bigger now. 

Finally, part of the problem of attributing the difference to population growth is that you have to explain the sudden shift in growth rates in the last 300 years vs. the previous 8,000+ years.