VMware normally creates “growable” disks, which start small and grow over time as you add data. However, you can also create “preallocated” disks that start at their maximum size. If you want to compact a disk later, you’ll need to convert it from preallocated to growable. Or, you may want to convert a disk from growable to preallocated for maximum performance.
Source: How-To Geek
