I think there is a rational argument for the US forces to contract their numbers of bases around the world. There are compelling arguments though that do stand up over time why they stay. The fact is, superpowers by their mere presence in a region actually do add stability. Whether it was the USSR or the USA, no one would attack one or the other in a region, and any regional conflicts that went on near the presence of the military of a superpower did so at their peril. The US has been in a lot of nations with their bases, mainly through accident or the aftermath of wars, and the presence of their troops being there has stopped hostile action in the territories they sit in. Russia or China didn't conteplate a move on Taiwan or Japan, and the Korean conflict has hit a defacto truce because mainly of the US Military being there. So there is a benefit to the "trip wire" of the US Army in that region.

I think Jag's point is spot on though. The US pays a lot more to maintain this presence than the multinationals based in America get back in business. In short, the US could probably spend the money better elsewhere if not for the geo-political benefits of the military being spread around the world.

As for the size of the US Navy, there is a VERY good reason it is that big. It makes for a more comfortable cycle and rotation for the sailors who have to maintain and supply the military presence around the world. What is more, the cost of having to build or borrow the ships if something does come up is a LOT. Just ask the people who had to write the checks in the UK in the run up and aftermath of the Falklands.

Large nations with influence, whether it be the USSR/Russia, or the USA, or China will maintain a large military because in the end, that is how they stay a large nation with influence.