The volume of watermelon pulp = 4/3 π (r-d) 3.

The volume of the whole watermelon = 4/3 π r 3.

The volume ratio of watermelon pulp to the whole watermelon

(4/3π(r-d)^3)/(4/3πr^3)=((r-d)/r)^3=( 1-d/r)^3

According to the result of (1-d/R) 3, because the skin thickness d is constant, the greater the r, the smaller the d/R, and the greater the volume ratio of watermelon pulp to the whole watermelon, so it is more economical to buy large watermelon.