Why is 1-ply toilet paper better for septic systems when you have to use twice as much per sitting?

I hear that when you have a septic tank, single ply TP is the way to “go.” However, you have to grab about twice as much paper in order to get the same results as a given amount of regular, two-ply paper. Wouldn’t that sort of negate the benefit of using the thin stuff? Please enlighten me!