What constitutes "responsible" defense spending depends on quite a lot of factors.
Bear in mind that the oceans on either side of the US already make defense rather expensive (have to be able to fight effectively on multiple fronts). Additionally, since the US's allies are overseas, there has to be a lot of emphasis on power projection. Even simply being the world's most prosperous country in absolute terms, the US is seen to have an obligation to defend and assist others.
Whether the *current* US defense budget is "responsible", I can't say. But even if it could be cut down, it would still be the biggest in the world.
By the time I've retired, we'll be talking about China and probably India the same way...of course, the rest of the world is far more enlightened than the U.S. and will have achieved eternal Peace, Love and Harmony by that time.