Volume Strain or, hydraulic strain, is defined as the change in volume per unit original volume, due to deformation. If V be the volume before the deformation and V + ΔV be the volume due to the deformation.
Then ΔV/V is the volume strain.
An air bubble at some depth in water has volume V. It is rising up. The pressure is less at less depth. Hence the bubble is relaxed a bit, its volume becomes V + ΔV. There is a volume strain of ΔV/V, which is positive.
We can squeeze a rubber ball in which case ΔV is negative and volume strain is ΔV/V is negative.