Sure, per capita GDP of Singapore is vastly higher, but here it's total GDP that's relevant, not per capita. Last I seen, GDP of Indonesia was 3 times that of Singapore.
Why not? The amount of weapons a country can buy is related to its total GDP, not per capita. The number of soldiers a country can field is related to its total GDP and actually goes up when per-capita GDP is lower, because you don't have to pay them as much.
