Virtual Machines can be useful for certain functions, and to solve problems without risking your main OS.
But there is fundamentally, no saving of HD space. If, however, it is only the intention to run a bare OS for problem solving then, of course, without any third party add-ons, it is a little smaller than the original. Remember a virtual machine also uses memory (RAM) when they are running. But, as well as the straightforward help factor, they can be used for several useful functions.