Just how big was the US Civil War?

From the Most Fearful Ordeal (Original Coverage of the Civial War by the writers and reporters of the NY Times with introduction and notes by James McPherson.

"At least 620,000 soldiers lost their lives from 1861 to 1865, constituting 2% of the American population. If the same percentage of Americans were to die in a war fought today, the number of American dead would be five and one half million." p.vii.

I got looking for WW II deaths and found this very interesting site. While the total number involved in WW II is staggering, as a percentage the death rate for the Civil War is a mind boggling 14.4%! Versus a still staggering 2.5% for WWII.