In a well known series "Crash Course World History", John Green claims that military spending has been fairly constant throughout history at between 3% and 5% of the state's GDP.
Is it really that universal? What about small states that had big, aggressive neighbors? Wouldn't they sacrifice more of their wealth to avoid being conquered? What about huge empires like Rome or 18th century Britain that had to defend from external threats, rebellion and were still expanding?
Is it even possible to estimate state expenditures and GDP of pre-modern states given their own difficulty in doing this and chronic problems with tax collection?
You probably misunderstood that guy, or otherwise I can't bring myself to believe that he believes such a thing.
It is absolutely false. It is widely accepted that military spending was the largest burden on Early Modern European states, while these days, military spending among the "world police nations" is ~ 5%, and in nations that don't have many enemies, it is usually below 1%.