I think Rumsfeld may have put just a little too much faith in his Special Operations people.
They don't like big masses of troops and probably told Rumsfeld it was unneccessary... which may have been true for phase 1 of the war. But what about securing an entire country?? I guess they just didn't think of that.
But the trend under Rumsfeld has been that military units have become smaller, more rapidly deployable and yet still remained effective. It's a big and neccessary change from the Army designed to stop massive wave of Soviet tanks running across Europe.