VMOptimizer is a specialised tool for optimizing VMWare Fusion virtual machines. VM files are actually a directory full of files, each of which holds data for part of the virtual machine’s disk. As such, the individual files can be fragmented; the internal structure of the VM file can be fragmented; and the filesystem stored within it can be fragmented.
Just defragmenting the files won’t fix this (it only fixes one of the three problems). Just defragmenting from the guest operating system won’t fix it either (again, it fixes one of the three problems).