It is generally true. Lee gave the Confederates leverage they would not otherwise had, and he and a few others made their smaller forces more effective than the larger forces which opposed them.
Even so, had the North simply pressed it's manpower advantage, they would have likely lost fewer men on both sides and attained victory much faster.
a manpower advantage is little use unless in the hands of a capable leader. The battle of Antietam is a very good example. The Army of the Potomac had almost a 50,000 man advantage in strength. McClellan managed to eke out a drawn battle, even with that advantage. Had someone like Grant or Thomas commanded the Army of the Potomac at the time, I could see the outcome to be very different.
Sheer numbers of soldiers do not win battles. The must be properly lead and equipped to be effective.