Verbose
Deity
Supply systems, as said.Why is it that in the Thirty Years' War, most armies were only around 20 or 30,000 strong, but the 18th century's wars (Spanish Succession, Quadruple Alliance, Austrian Succession, Seven Years'), armies were up to 100,000 strong? Surely the population in Europe didn't change that drastically in 50 years.
But it's not entirely true that field armies ranged in the 20-30 000 span. Those were the large armies deployed at the outset of the conflict. As the war dragged on, and as the armies passed through areas that had been devastated by war before, field army sizes shrank.
Thus Gustavus landed with something like 30 000+ men in 1630. But already by 1636, when Johan Banér stepped up to revive Sweden's flagging martial reputation, he had to make do with armies in the range of about 8000-14 000 men. The only time when he actually got to play with an army of the size of the initial conflict was when temporarily hooking up with his French allies.
And these smaller armies from the later stages of the TYW also displayed a much greater reliance on horses. Ideally the entire damn army should be on horses, not so much to fight as such, but to move around. Being able to quickly move through or away from a devastated area where the army couldn't be fed could be key to success or failure. The situation was such in Germany that military action ended up dependant on where troops could be supplied at all.