6" / 4" = 1.5. The cross-sectional area increases with the square of the diameter, so a 6" pipe has cross-sectional area (3 / 2) **2 = 9/4 = 2.25 that of a 4" pipe.
Same reason that a small increase in monitor diagonal measurement makes more of a difference than you'd initially think.
For the math-challenged...
Same reason that a small increase in monitor diagonal measurement makes more of a difference than you'd initially think.