One sees how the Spartan army was always dependent on its neighbors for fighting strength (Periokoi and allies). It’s easy to forget that Spartiates came from the combined town of Sparta which was not very large. When you factor in the use of Spartiates for other purposes, like occupation and administration of conquered territories, and the necessity to keep some of the fighting strength in reserve, the final number eligible for battle at any one time was quite small.
This review shows us a shortage of Spartiates after the earthquake, growing to a larger number by Mantinea, and then back down by Leuctra. It is surprising to me that Sparta was willing to take on so many in the Peloponnesean War. Maybe mistrust of Athens was the motivator that overcame Sparta’s concerns about the size of its army. Of course, the trump card was the Peloponnesean League, which extended the military strength of the Spartan nation.