What is A Force of 10K Going to Achieve That one of 150K couldn't?

Should the Military Pull All Forces Out of Afghanistan After 2014? - The Daily Beast:

"The White House release indicated that if the BSA is signed, the forces left in Afghanistan would be given “a limited post-2014 mission focused on training, advising, and assisting Afghan forces and going after the remnants of core Al Qaeda.” Many reports suggest senior military and diplomatic officials believe that a force 10,000 US troops is the right size to accomplish those missions. One senior officer said the future in Afghanistan would be “grim” if no US troops remain after 2014."