We just can't build an Afghan army big enough to control the country:
Coalition forces continue to face massive challenges in creating a modern, dependable, loyal Afghan army. It's also becoming evident, and frustrating, that the magnitude of the task is greater than it was in Iraq. That's because Afghanistan and Iraq have different military traditions and histories that have made it easier to build stable security forces in Iraq. While the Iraqis are beginning to consistently operate on their own and even to master some of the more complicated tasks of warfare, like air assault operations, coalition forces are still grappling to introduce some basic modern concepts to the Afghan National Army (ANA), after seven years of effort.
And even if we can build it, Afghanistan could never afford to pay for it. Will NATO continue to pay for an Afghan army of a quarter million strength after their troops are gone? I doubt it. And I doubt we'll do it.
Build a small but good army that the central government can actually afford and which doesn't strain the available human resources; and build up regional and local defense forces that can hold their local areas against most threats.
We have to find a security solution based on the human and financial resources we have and not design an idealized security solution that depends on developing the resources to support that ideal solution.