But your total defense spending should be informed by your enemies' total defense spending, not necessarily by the size of your economy. From that perspective I find the above statement pretty illustrative.
To be fair to the U.S. every powerful country makes a lot of enemies and throughout history there has always been some country that spends a large portion of their money to protect their interests and expand their power base. Egypt, Rome, China, Spain, Britain, France, Germany, The Ottoman Empire, Persia, etc. There's always someone globally at the top who needs to spend a large portion to protect entrenched interests and they inevitably fall because those entrenched interests forget that a country only states at the top if it's willing to make new and more powerful interests.