I was trying to compare the military spending of the Roman empire in the 1st century with modern military spending and how much of a burden that was to Roman people. The Wikipedia page of the economics of the Roman army states that the military spending was about 2.5% of the Empire GDP.
That lead me to search for the figures estimated for the GDP of the 1st c. Roman empire. I noticed that the government spending was very low compared to modern spending. For example, this paper estimates the GDP and states that the government spending was about 5% of the total GDP. Comparing that to the US now with about 40% of GDP as government spending, the difference is very obvious.
So my question here is why weren't the Roman empire able to maintain a government spending level similar to modern governments ?