I came across a statement that whenever any stretching or compressing is done on any object, its change in volume is always greater than $0$. This means that density always decreases (mass remaining constant) or, it remains constant for purely incompressible objects but it cannot increase.
Why is that? Why can't the density of an object not increase on applying any sort of strain on it? Here is the context in which I came across it, Poisson's ratio
Edit: Chill guys, I do not study from this link or blogspot. I googled the question and got this link among the many that actually had some proof, although wrong. That's why I asked it here for clarification.