[GravitasFailure]

Much of the time the size of your army was determined by the number of able bodied men that lived on your lands, and they were armed with whatever weapons and gear they brought with them. Feeding and housing them wasn't that much of an expense, either, since armies would often confiscate whatever food they needed from farms and towns along the way (leading to the 3rd amendment of the US Bill of Rights.)

It wasn't until standing armies with standardized weapons and gear became more of the norm that a smaller, better equipped army was really even an option, and by that point firearms technology (and artillery) had progressed enough that armor wasn't very useful. Even then, soldiers wearing a cuirass into battle were still a thing up until WWI, so it's not like the practice died overnight.