The question is somewhat ambiguous but I will attempt to help with what I suspect is the thought behind it.
Since WW2 the United States has felt that the military should be able to force project anywhere, anytime, in any conditions. While this is an admirable goal, it is expensive and requires a large force. The second issue is that we have since WW2 felt we would have to fight at least two simultaneous regional conflicts. This does not necessarily mean Iraq and Afghanistan. It really means something like the Middle East and sub Asia or Europe and the pacific. To make this work the military has specific units trained for contingencies in certain land mass types. I’m thinking of units like the 10th Mountain in Ft Drum or the Air Force units stationed in Alaska. This increases the size of the military.
Additionally, thinking about the Navy, if you want a carrier, you really want a carrier, but the carrier has to be protected by a given amount of support units. And how about the logistics trail to get beans, bullets, and bacon to the fighting man at the front. All this is expensive and increases the size and composition of the military. Let me cite one big change since WW2 that reflects this thinking - In WW2 the army had railroad units, truck units, and large stevedore units to move men and equipment from the point of embarkation to the front. In Iraq currently, quite a bit of the trucking is done by civilian contractors (at a cost) to keep the size of the military down. However on this specifically it has had the interesting side effect of requiring more military “protection” than the old army trucking units which provided their own. I’m not saying this is good or bad but pointing out how even small changes have ripple effects on military manpower.
If we as the American people though our elected officials (as inept as most are) decide that the requirements for the military can be scaled back – there can be a reduction in the size. But the military in war is not something you want to ask to do more with less. You want to show up with overwhelming firepower and kill everything in site.
Tomorrow is the most important thing in life. It comes into us at midnight very clean. It’s perfect when it arrives and it puts itself in our hands. It hopes we’ve learned something from yesterday. - John Wayne
While I agree with the general sentiment of “we want to project force anywhere, anytime,” I think a lot of people are hung up on a fantasy confrontation with Russian and China still. Attacking anything other than a third-world country like Iraq or Afghanistan is either going to be a handful of special-ops guys or a volley of nukes. There’s no way in heck we’re ever going to be fighting a column of russian tanks, or facing down a million chinese with ak-47’s from across the battlefield. We’re going to turn them into a glass parking lot (and they’ll try to do it to us in return).