According to the maths I learned at school, the circumference of a circle is 2π times its radius. So if I add an overgrip of thickness 0.6mm to the handle of my racket, it should increase the circumference of the handle by approximately 0.6 x 2 x 3.1 = 3.72mm, which is roughly 1.2 grip sizes. However, it is commonly stated that an overgrip adds around 0.5 size (e.g. see Increasing Grip Size). Can someone explain where my reasoning is going wrong?