Adding and removing files in VirtualBSD causes its disk usage to increase/decrease accordingly but, due to the nature of the expanding virtual disk, the virtual machine itself will keep using more and more of your computer's disk space.
In other words, no matter how many files and folders you delete in VirtualBSD: once you've added them, the virtual disk won't shrink back automatically. Still, you can recover the unused disk space by following these steps:
1) From the Applications menu select System, then Shrink Disk;
2) Click on the Shrink tab, then click on / to select it. Finally, click on the Shrink button in the bottom left corner:
3) Confirm the operation by clicking on Yes:
4) A somewhat lengthy process will start and this bar will be (slowly) filling up:
5) Now that the preliminary work is done you can start the shrinking proper by clicking on Yes:
Note: This operation will take a few minutes and the Virtual Machine will be frozen for the whole duration.