Factors that Strategypage notes are:
The varying costs of supporting troops, which allows poorer countries to have an effectively higher defense budget because of lower personnel costs. In America, paying for troops and feeding and housing them, for example, is much more expensive than in China or India.
How long you've been spending sufficiently on defense. China is just starting to buy advanced weapons and so even if some approach ours in capability, we've built up a lot more because we have consistently--with ebbs and flows, of course--built up our military.
Your military's mission. If you plan to attack neighbors, your costs are higher for logistics capabilities. The farther you have to go, the more expensive it will be.
Your quality of leadership in officers and (if you have them) NCOs (the sergeants). If these troops are competent and honest you have a better military. If you have corruption or a jobs program, your military isn't as effective.
Unstated but related to the attacking versus defense is America's geographic situation. It isn't only an attack versus defense issue. We have to go a long way to defend any ally except Canada and Mexico. This capability to move and supply a major force far from home costs us a lot of money.
Other unstated factors:
Precision weapons are an added cost. We do this to reduce friendly casualties, collateral damage, and even enemy casualties. If you want to save money you can just drop dumb bombs the way Russia does in Syria.
Or the way terrorists kill--this is grimly cost-efficient, you must admit:
At least 29 children enjoying an Easter weekend outing were among those killed when the [jihadi] suicide bomber struck in a busy park in the eastern city of Lahore, the power base of Prime Minister Nawaz Sharif. Pakistan is a majority-Muslim state but has a Christian population of more than two million.
Training also costs money. This is related to leadership quality, but it costs money to train a military and keep it capable of fighting effectively--and without committing war crimes. You can save money by cutting training, but you will pay in blood--friendly and innocent, and even enemy by dragging out a war--later.
America also maintain war reserve stocks of ammunition that few other Western countries bother to maintain because of the expense. In part, these reserves are meant for our allies which allows them to focus more of their spending on weapons, counting on us to resupply them. Even when allies pay for this resupply, we bear the ongoing cost of maintaining the stockpiles.
Nukes cost a lot of money to build, maintain, and secure, but don't affect your conventional military capabilities--other than deterring nuclear and other WMD strikes on them. So if you have a robust nuclear deterrent, that money doesn't really say much about your warfighting capabilities.
So we need to spend more money than other countries we might fight just to be on an even playing field with them in actual battle and to fight as cleanly as possible.
And when you add in the desire to win while minimizing casualties, costs go even higher.
If you want American spending lower in line with other countries with completely different strategic situations, tell me that it is acceptable to have more deaths in war, lose wars, abandon allies, or watch evil strengthen or atrocities on a grand scale take place without trying to stop them.
UPDATE: Related (tip to Instapundit). A move to reform:
Partly as a result of the cumulative, multi-year effects of sequestration, and partly because personnel and O&M (operations and maintenance) costs continue to outstrip annual overall defense budget growth, both military end strength and force posture have declined since the final year of the George W. Bush Administration. In particular, overhead and support now comprise over 40 percent of total Pentagon spending—some $240 billion out of a base budget of approximately $550 billion. As such, DoD’s overhead figure is more than twice the combined total defense budgets of France and the United Kingdom. The cost of DoD headquarters alone amounts to over $40 billion, more than the entire German defense budget.
Reform issues are really out of my realm of familiarity. That overhead and support is necessary to build (and expand), send, and sustain a capable military overseas. So O&M is needed at some level.
But I can't say what is waste and what is necessary.
What I can say is that when our end strength is going down in a dangerous world, we need some type of reform to restore our military.
If only all government could benefit from that.
Do read it all. A lot of reform is necessary. Let me comment on two things that I can address.
Since 2001, the ranks of DoD civilian personnel have increased by over 15 percent, or some 100,000 individuals, while military personnel has declined by about 5 percent.
Because the Army could not add end strength, to expand the Army to fight in Iraq the military transfered tens of thousands of military jobs to the civilian sector, freeing up slots that could be used to add fighting troops to the force.
That's one reason that reducing the Army to pre-2001 levels has not required an even steeper decline in combat brigades.
As for reducing the bases? Good luck. Surely necessary, this money-saving effort will see resistance from the states where the bases are located. When I worked for the state legislature, I wrote a number of resolutions to defend Michigan facilities from the BRAC process cutting board. So that is difficult.
Something needs to be done.