For a constant hydraulic stress on an object, the fractional change in the object’s volume $(∆V/V)$ and its bulk modulus (B) are related as  

When strain is small the ratio of the normal stress to the volume strain is called the bulk modulus of the material of the body

$B=\frac{Normal stress}{volume strain}$

$B=-\frac{∆P}{∆V/V}$

$B\propto \frac{1}{{\displaystyle \frac{∆V}{V}}}$

$\Rightarrow \frac{∆V}{V}\propto \frac{1}{B}$