Overly simplified answer: Surface area of places they protect and the need for the most modern weaponry/training.
Unfortunately, America farms out their military might -- both personnel and armaments -- to other countries... and, in some cases, account for the majority of some nations' defensive forces. Couple that with sheer size of our own nation, R&D for military equipment, the crushing financial burden of paying every service member who is in -- to include those who qualify to gain remuneration when they leave active service for the remainder of their natural lives -- and ludicrous deals made by special interest groups to supply the military with gear, it's not hard to see why. It's not a one-to-one increase; our costs are exponentially higher.
A good portion of our funding could by instituting several changes. We might again consider simply charging nations for our services. If called upon to provide military support, in any capacity, we bill the nation that is requesting such support. We could further open up supply contracts to a wider market... the whole "$100 for a hammer; $200 for a shovel" meme is fairly accurate... and fiscally retarded. Similarly, contracting out positions that could and should be filled with green suiters would be more cost effective and yield security benefits.
Basically... the answer to your question is "it's nuanced" but to question how and why your government is spending OUR money is salient and critical for federal accountability. Good on ya for asking the question. I suggest you do some further research because the answer(s) will both impress and shock you in equal parts, I'd imagine.
Caption this Meme
