... is that the transition to contractors was driven by authorized end strength, the Congressional limit on the number of people in the active duty components.
As we stand today that is correct

The authorized end strength of the Army is 512,400 (authorized for 2009). The Army wants to get maximum combat power out of that number. The brass has decided (correctly, in my opinion) to identify areas where they can hire civilians to perform certain activities, and free up another troop slot for combat power.
Yes again

That is the reason every company doesn't have its own mess section. Its the reason so much of the higher echelon maintenance/repair is performed by contractors. It's the reason a lot of the logistics (i.e. transport) is done by contractors. Having contractors do a lot of that work frees authorized slots for combat power.
Not exactly. The removal of mess sections, and maintenance sections to create light infantry divisions was all part of AOE as described below:

Army of Excellence
The centerpiece of the reorganization, the light infantry division was a 3-brigade organization with 9 battalions of straight foot-infantry, with a strength eventually set at 10,800 men. Deployable in approximately 550 C-141 airlift sorties, it was oriented specifically to contingency actions worldwide where response in the first days of a crisis was critical. Lacking armor and heavy howitzers, the division was structured on shock tactics rather than sustained firepower. Based on the historical lessons of World War II, force designers incorporated "corps plug" augmentation forces into the scheme to make up for the lack of firepower and logistical capability. By concept, an early-arriving light division could buy time for heavier forces to follow. The light division had a secondary mission of reinforcement of heavy forces in scenarios and terrain where it could be more effective than those forces - in cities, forests, and mountain areas. Many light infantry division capabilities were austere. The division - contingency focused - was conceived and approved as a hard-hitting, highly trained, elite light force, with high esprit and cooperation essential to its success. The design went through a successful certification process in the 7th Infantry Division (Light) at Fort Ord, supported by the TRADOC test organizations, during 1984-1986.
There are two trends here: one was to reduce tail by either removing the function and replacing it with a partially technological fix as in MREs with T-rations served via micro waves. Or maintenance with so-called plugs from above; we all wondered what would happen when the supply of said plugs was expended.

This was going on well before then SecDef Cheney put the use of contractors into hyper-drive; see Public War, Private Fight? The United States and Private Military Companies, Deborah C. Kidwell. Paper #12. for a pretty good discussion of that.

The other trend was reduction of the military and removal of those functions in favor of "teeth". As you say, fewer troops means keep the shooters.

A third but somewhat buried trend in all of this dealt with breaking with the Weinburger/Powell/Myers changes to the military that required full mobilization of reserves and NG. Giving those functions over to private companies reduced societal and political costs (in theory). This dovetailed nicely with transformational rhetoric about the intrinsic value of rapid deployment, none of which took into account the costs of an extended war effort.

Tom