Murdoc Online notes that the new aircraft carriers planned for the British Navy, will have about a third of the ship's crew of existing ships that size from about 2000 to 800 crew, and will also greatly reduce air crew from about 600 air crew to considerably less than that number.
Similar crew reductions are foreseen for the American Navy. The frigate sized Littoral Combat Ship, will also have about a third of the ship's crew of existing frigates, and just about every other new warship design is looking at similar reductions. As existing American aircraft carriers are refurbished from time to time, there is a new focus on making modifications that reduce the number of crew required.
The Air Force is also expecting to be able to lower its per aircraft maintenance crew needs, largely because the F-35, which is about to enter service and replace the F-16 that has been the Air Force's mainstay since the 1970s, which will replace its existing aircraft, was specifically designed with maintenance costs in mind.
In other words, simply by replacing existing ships and aircraft with new ones, on a one for one basis, you would expect to see dramatic reductions in the personnel size of the Air Force and Navy.
In part because the new systems are more effective than the old ones, and in part because these new high tech systems are so immensely expensive that Congress doesn't want to pay for them all, the replacements will not be one to one. For example, proponents of the new F-22 fighter for the air force have sold the program to a great extent on the claim that it can easily defeats multiple enemy aircraft even when outnumbered. Similarly, more accurate guided bombs making existing bomber aircraft more effective, since the same payload can be directed usefully at more targets. The usual comparison of "smart" v. "dumb" bombs looks just at how many targets can be hit per dollar of ammunition cost, a comparison that is often a toss up, but few of these comparisons note that hitting the same number of targets with fewer "smart" bombs reduces the number of very expensive bomber aircraft that must be purchased as well.
Even if the replacements required the same crews, and even if the replacements were no better than what came before, it would still make sense to reduce the size of the U.S. Air Force and U.S. Navy. Simply put, both the U.S. Navy and the U.S. Air Force, particularly when viewed not in isolation, by as one piece of a coalition that any major war would be fought with, are overwhelmingly superior to anyone else on the planet at what they are designed to do. With the collapse of the Soviet Union, the U.S. Navy's blue sea capabilities have become unrivaled and largely unnecessary. The will never be a repeat of the World War II campaigns in the Pacific and the Atlantic. Even the biggest proponents of the battleships that were recently phased out of U.S. service don't want them for the purpose for which they were designed, ship to ship combat with big naval guns, they want them to serve as artillery batteries for Marines storming some distant beach.
This doesn't mean that there aren't gaps in both the U.S. Air Force and the U.S. Navy.
The Air Force is still shirking its obligation to provide close air support to ground troops, neither the refurbishing of 1970s era A-10 Warthogs and AC-130 gunships, nor the new F-35 (which was sold as a replacement for the A-10 among other aircraft), are meaningful long term answers to that inadequacy. The Navy is sorely lacking in its ability to operate in coastal waters (both to support ground troops and to engage in other conflicts there), and has an inadequate ability to clear large swaths of coastal territory of advanced diesel submarines and anti-ship mines (the IEDs of the sea).
And, the U.S. military, in a job that has traditionally been allocated to the Air Force and Navy, has a grossly inadequate ability to rapidly deploy large numbers of troops over great distances. Some combination of air ships, airlift aircraft, and high speed sealift, are sorely needed to meet this gap.
The Army and Marines aren't immune to this trend either. For example, automation of artillery batteries, can reduce the number of soliders needed to carry out that function, and surveilance drones can replace much more crew intensive surveilance helicopters and foot patrols.
But, none of this replaces the grunt. Boots on the ground make a difference in places like Iraq, and human soldiers capabilities aren't signficantly improved from half a century ago, with the exception that they are far less likely to die from their battle wounds due to improved body armor and medical care.
Thus, on a percentage basis, it is almost certain that the military a generation or so from now will, ironically as a result of technological advances, have a far larger share of fairly low tech infantry, and a far smaller share of almost everything else.