Westros’s armies make up about 1% of its population, right? Is that a function of its feudal society (and a different, more centralized society would have a higher number), or the expense of raising an army, regardless of time period (and so even if Westeros moved to a centralized absolute monarchy, the number of troops would not rise)?

It’s a rough estimate of the size of armies that premodern societies could keep under arms. Even Ancient Rome at its height of wealth and administrative organization couldn’t keep more than 1% of its population.

The basic problem is that in premodern economies, the limited productivity of agriculture means that there’s a limited surplus you can use to feed non-farmers and that there’s real tradeoffs between your army and your agricultural labor force. 

Add onto that serious problems with logistics and organization – you need systems in place to buy enough food to feed the army, transport it from farm to (army) table, store it so that it doesn’t go bad, and then distribute it. And that’s just to feed the army – you need to do the same thing for arms, armor, equipment, clothing, and pay (especially if you don’t want the army to desert or mutiny). 

That’s a heavy lift for any state. Rome was exceptionally good at administration, but even it was quite a strain on the system – you can especially see the problems they ran into with paying the army vs. keeping the coinage at a decent value, hence why the Roman state eventually shifted to systems of requisition and payments in kind, which eventually lead to quasi-feudal systems which would eventually give rise to the feudal army. And so army sizes dropped – ancient Rome had 700,000 men under arms across the whole of the Empire, but medieval states were lucky if they could get their army up into the tens of thousands. 

However, around the 16th and 17th centuries in Europe, you had something that historians call the military revolution, or sometimes the early modern military revolution, where suddenly armies increase in size into the hundreds of thousands. And then by the time of the French Revolution, for the first time you’ve got nations with more than a million men under arms. Part of this had to do with changing productivity – there were persistent improvements in agricultural productivity (the so-called agricultural revolution), there were improvements in transportation which meant that you could transport goods faster over longer distances while losing less of your goods to spoilage and wastage, and there were major improvements in bureaucratic administration, which allowed armies to organize their supply systems effectively. 

So no, it’s not a natural limit.