The radius is half the diameter
So to make it even you would have the diameter of 15 kilos for the single impact; and then 30(2) = a diameter of 60 kilos for the complex impact.
Circumference is diameter multiplied by pie
So now
Single impact: 3.14(15)= 47.1
Complex impact: 3.14(60)= 188.4
188.4 / 47.1= 4
Making the complex impact’s circumference 4 times greater than the single impact’s circumference.