How to keep the volume of an obj when reshaping?

I want to make a script that changes the obj's X & Y when altering the Z to keep the same volume.

My problem is that if I for instance have that when I set Z, the X & Y change accordingly but how do I enlarge (or shrink) X & Y equally?

What I thought of was to do this:

 // V = Volume
 // V = X*Y*Z
 
 X = V/(Y*Z);
 Y = V/(X*Z);

But then I realized that that doesn't make any sense because when I run the first line, Y still has it's same size as before and has yet to be altered.

So how do I make a formula that calculates X & Y equally?