I'm playing a game now as Spain, in which the Ottomans got wrecked early on (seemingly they fought Austria-Hungary/Venice/Pope others and the Mamluks at once) but they still have over 200k men. I don't know how that is possible. They've started eating the Commonwealth now, but I've allied the Ottomans because I have a northern border with them (Austria/Hungary PU) and don't want to fight them whilst I'm colonising and eating land in Asia and Africa.
They actually attacked Austria-Hungary whilst I was PUing them (claim throne, they had a Trastámara) and I had to fight them after winning the war, taking leadership, and they mopped the floor with my armies, I was just lucky their war exhaustion was high from the previous war and I was able to white peace them by fully occupying their ally Bohemia. Again, not sure their armies should be THAT OP to easily beat me in defensive mountain forts.
I think my real problem is just the amount of men that they can field. Despite the Mamluks, Commonwealth, Russia being bigger, they still have more men and an army equal in size to me, a colonially rich Spain (Own most of the Americas with trade companies in Africa and soon in Asia) that's taking quantity ideas.