> Armor is an occasional expense -- purchased once, repaired infrequently. Soldiers are a constant expense -- they have to be paid, fed, housed, etc.
At the end of the article the author notes that armor would actually have to be repaired quite often, and that doing this for large armies was likely impractical. Further, while a small armored force might be able to defeat a larger force in a single battle, there are definitely advantages to having more warm bodies: you can cover wider areas without gaps, you have more manpower to build fortifications, you can rotate people through stressful parts of soldiering, etc. It doesn't necessarily follow that the massive upfront cost is justified, especially if you can get a lot of the benefit by only having a much smaller number of elite units equipped with such armor.
Then there are further logistical concerns - soldiers don't just need to survive battles, they need to complete objectives, which means a lot of marching to locations and moving around to deal with situations. While craftsmen of the period were capable of producing bulletproof armor, it was nevertheless getting heavier - not so heavy that you couldn't move around in it, but heavy enough that you might move slower. As per the article, the trend was toward lighter, more maneuverable units armed with a lot of firepower. While heavier units could still win from time to time, that doesn't necessarily mean they were better.
> So then, why did the size of armies grow?
> If firearms were ineffective against proper armor, why did their use spread? I've always understood that the advent of firearms and their effectiveness led directly to knight's loss of dominance on the battlefield but this article strongly implies otherwise. So if it wasn't the firearms, what was it?
Armies weren't growing because of the adoption of firearms, firearms were being adopted because armies were growing. Firearms were relatively cheap (especially compared to the armor required to protect against them), and compared to weapons of similar range and capability they required little training. And if you look at the evolution, it's basically you start with an army of pikemen, then you have an army of pikemen where they strap a gun barrel to their pike to get one shot off at the start of the engagement, then you have pikemen where the pike is a musket and they plug it with a bayonet, and eventually you have socket bayonets that allow the soldier to have a perfectly functional musket and pike at the same time; and at that point we start calling them musket men instead of pike men. Firearms were adopted because they were a cheap and seamless way to boost the effectiveness of a fighting force, not because they were a counter to any particular force on the battlefield.
At this point in time political organization had increased to the point where wars were being fought between nation states, and there simply weren't enough people who had been training for their whole lives to be longbowmen or lancers to fill the ranks. Really this a return to form, in more politically organized areas and time periods, such as China or the Roman empire, armies in the hundreds of thousands were not uncommon. Medieval Europe just had a long period where no one could consistently muster a force that could resist a good sized group of knights on horseback. Heavy cavalry were still effective, as demonstrated by the Winged Hussars, but you needed them to scale to appropriate numbers and use them appropriately in the new warfare framework.