Why is the relationship between radius and volume non-linear?

Answer 1

radius is measured in units, linear units, 1 dimensional length

area is measured in square units, 2 dimensions

Volume is measured in cubic units, 3 dimensions,

to relate 1 dimension to 3 dimensions is not a linear relationship

For a sphere, Volume = (4/3)pi(r)^3, which is a cubic relationship, not linear.