fbasilico wrote: But when i'm running it i pay attention to my local storage and i can see that sometimes decrease.
It is quite normal and expected that as applications are opened, the amount of free disk space on the internal drive can decrease. It actually would have been more helpful from a diagnostic/troubleshooting perspective to have actually quantified the differential! 
As an example, with VMware Fusion already open I start a VM from an USB 3.0 Drive that has 8 GB of RAM allocated to it and on my early 2013 MacBook Pro the amount of Used space increases 153.6 MB and when the VM is shutdown it decreases by the same amount. Now I consider that an insignificant amount and I wouldn't give it a second thought, although if it was say.. 10 times that (or more) then I'd want to know why.