It's not that disproportionate. Most militaries are funded within 2%-6% of GDP. America's economy is just so enormous that the spending seems ridiculous. Yes, there is a lot of waste, a LOT OF WASTE, but it's a small price to pay to feel safe and secure from foreign militaries when one has the benefit of, as the economist put it: "friends to the north and south and fish to the east and west."
The thought that a war could ever occur on our own soil is simply something most Americans don't even contemplate, unless you've spent one too many evenings polishing your favorite firearms and rewatching "Red Dawn."