Question 973473
They are both functions of the cube of the radius or even the cube of the diameter for that matter, since one is a linear function of the other.

V=(4/3) pi r^3

If you do volumes of both, the (4/3) and the pi cancel

Do ratios of r^3

2.5^3/1.5^3
=15.625/3.375
=4.63

Note:
The ratio of their respective diameters would be 125/27, which would be the same