Say during the civil war the southhad the same amount of people as the north. Would they south win??
Probably not. The South didn't have the industries, the money, or the moral imperative to win the Civil War.
During the American Civil War, the determining reason the North won was superior industrial capacity. In a long war, especially a war of attrition (look that word up), industrial strength will favor the winner.
Now in a Short WAr, industrial strength wouldn't matter as much, and one can speculate forever on that. AT the beginning, the South had several advantages: They had spirit, good military leaders, and loyal soldiers.
Other factors beside population base and industrial strength is sea trade (meaning a Navy to protect ports), and the stamina of the civil population to standfast in the wake of hardship.
I hope this helps.