So you not only installed Workstation, but created a virtual machine. The virtual machine’s disks will take up additional space on your disk as you install the operating system and use the virtual machine.
The option to split virtual disks into multiple pieces typically provides the lowest use of disk space. The pieces start out small, and grow as you use disk space in the VM. Other types of virtual disks consume more disk space (with the option to fully allocate the disk consuming the entire amount of configured space).
You may want to review the documentation to get a better understanding of how Workstation works.